Start using nix-fast-build in github action CI

[henrirosten]

• Move pycodestyle, pylint, black, and format checks from task.py to ghaf-infra flake .#checks
• Remove all redundant checks from task.py
• Introduce nix-fast-build.sh helper to run relevant flake builds with nix-fast-build to execute all evaluation/build operations completely on the remote builder. Add relevant dependencies to nix devshell.
• Start using nix-fast-build.sh from github action workflow test-ghaf-infra.yml: run all checks and build relevant x86 and aarch targets on remote. This adds some new checks too: the workflow now also builds all azure nixosConfigurations, as well as runs the .#checks on aarch64.

Note: after this change, cachix cache on the test-ghaf-infra.yml workflow is used only to cache the ghaf-infra devshell dependencies. This is to make the test-ghaf-infra.yml faster by not having to download the build closure to the github-hosted runner. Rationale is that local builds of all the ghaf-infra nixosConfigurations are likely rare, and when required, nix-fast-build.sh can be used locally too (in which case the remote builder's local nix store would be used as a 'shared cache').

Example run of test-ghaf-infra.yml workflow with the changes from this PR: https://github.com/henrirosten/ghaf-infra/actions/runs/11854339487