feature: make it possible to override generate dirs via env

[fgimian]

Description

This change simply makes it possible to optionally specify COMPLETION_DIR or MANPAGE_DIR via environment variables to specify the location where completion and manpage files are created. This is especially handy when using cargo install and is a similar approach used by hyperfine.

This also makes sure the docs align with the implementation as build_process.md states the following:

• Build a release build with:

  • --features deploy, which enables only crates needed for release builds.

  • --locked to lock the dependency versions.

  • The following env variables set:

    • BTM_GENERATE: true
    • COMPLETION_DIR: "target/tmp/bottom/completion/"
    • MANPAGE_DIR: "target/tmp/bottom/manpage/"

Issue

Partially addresses #1654 for those using cargo install

Testing

Windows

~\source\bottom on  allow_generate_customization_via_env is 📦 v0.11.0 via 🦀 v1.85.0-nightly
🕙 [ 08:18:10 PM ] ❯ ls C:\Users\Fots\Downloads\bottom-generation
Get-ChildItem: Cannot find path 'C:\Users\Fots\Downloads\bottom-generation' because it does not exist.

~\source\bottom on  allow_generate_customization_via_env is 📦 v0.11.0 via 🦀 v1.85.0-nightly
🕙 [ 08:18:10 PM ] ❯ $env:BTM_GENERATE = '1'

~\source\bottom on  allow_generate_customization_via_env is 📦 v0.11.0 via 🦀 v1.85.0-nightly
🕙 [ 08:18:13 PM ] ❯ $env:COMPLETION_DIR = 'C:\Users\Fots\Downloads\bottom-generation'

~\source\bottom on  allow_generate_customization_via_env is 📦 v0.11.0 via 🦀 v1.85.0-nightly
🕙 [ 08:18:15 PM ] ❯ $env:MANPAGE_DIR = 'C:\Users\Fots\Downloads\bottom-generation'

~\source\bottom on  allow_generate_customization_via_env is 📦 v0.11.0 via 🦀 v1.85.0-nightly
🕙 [ 08:18:16 PM ] ❯ cargo build
   Compiling bottom v0.11.0 (C:\Users\Fots\source\bottom)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 4.02s

~\source\bottom on  allow_generate_customization_via_env is 📦 v0.11.0 via 🦀 v1.85.0-nightly took 4s
🕙 [ 08:18:22 PM ] ❯ ls C:\Users\Fots\Downloads\bottom-generation

    Directory: C:\Users\Fots\Downloads\bottom-generation

Mode                 LastWriteTime         Length Name
----                 -------------         ------ ----
-a---           7/01/2025  8:18 PM           5155 _btm
-a---           7/01/2025  8:18 PM          10734 _btm.ps1
-a---           7/01/2025  8:18 PM           9025 btm.1
-a---           7/01/2025  8:18 PM           4110 btm.bash
-a---           7/01/2025  8:18 PM           5477 btm.elv
-a---           7/01/2025  8:18 PM           4315 btm.fish
-a---           7/01/2025  8:18 PM           4457 btm.nu
-a---           7/01/2025  8:18 PM           7403 btm.ts

Linux

fots@Fotsies-PC:~/bottom$ ls -l ~/
total 4
drwxr-xr-x 16 fots fots 4096 Jan  7 20:33 bottom
fots@Fotsies-PC:~/bottom$ export BTM_GENERATE=1
fots@Fotsies-PC:~/bottom$ export COMPLETION_DIR=/home/fots/bottom-generate-completions
fots@Fotsies-PC:~/bottom$ export MANPAGE_DIR=/home/fots/bottom-generate-manpage
fots@Fotsies-PC:~/bottom$ cargo build
   Compiling bottom v0.11.0 (/home/fots/bottom)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 3.51s
fots@Fotsies-PC:~/bottom$ ls -l ~/
total 12
drwxr-xr-x 16 fots fots 4096 Jan  7 20:33 bottom
drwxr-xr-x  2 fots fots 4096 Jan  7 20:34 bottom-generate-completions
drwxr-xr-x  2 fots fots 4096 Jan  7 20:34 bottom-generate-manpage
fots@Fotsies-PC:~/bottom$ ls -l ~/bottom-generate-*
/home/fots/bottom-generate-completions:
total 60
-rw-r--r-- 1 fots fots  5241 Jan  7 20:34 _btm
-rw-r--r-- 1 fots fots 10921 Jan  7 20:34 _btm.ps1
-rw-r--r-- 1 fots fots  4132 Jan  7 20:34 btm.bash
-rw-r--r-- 1 fots fots  5577 Jan  7 20:34 btm.elv
-rw-r--r-- 1 fots fots  4418 Jan  7 20:34 btm.fish
-rw-r--r-- 1 fots fots  4548 Jan  7 20:34 btm.nu
-rw-r--r-- 1 fots fots  7534 Jan  7 20:34 btm.ts

/home/fots/bottom-generate-manpage:
total 12
-rw-r--r-- 1 fots fots 9150 Jan  7 20:34 btm.1

• Windows
• macOS
• Linux

Checklist

If relevant, ensure the following have been met:

• Areas your change affects have been linted using rustfmt (cargo fmt)
• The change has been tested and doesn't appear to cause any unintended breakage
• Documentation has been added/updated if needed (README.md, help menu, doc pages, etc.)
• The pull request passes the provided CI pipeline
• There are no merge conflicts
• If relevant, new tests were added (don't worry too much about coverage) - N/A, only changes the build.rs script 😄

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 53.82%. Comparing base (c9ffc41) to head (16e678b).

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1658       +/-   ##
===========================================
+ Coverage   41.32%   53.82%   +12.50%     
===========================================
  Files         109      109               
  Lines       17900    17900               
===========================================
+ Hits         7397     9635     +2238     
+ Misses      10503     8265     -2238     

Flag	Coverage Δ
macos-14	37.26% <ø> (ø)
ubuntu-latest	56.19% <ø> (+13.14%)	⬆️
windows-2019	37.18% <ø> (ø)

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