Compress generic-x86_64 image

[remimimimimi]

Description of changes

Checklist for things done

Binary images of all other targets are compressed already during the build phase. This makes downloads significantly faster. Compress also x86_64-linux.generic-x86_64-debug image in the build phase.

• Summary of the proposed changes in the PR description
• More detailed description in the commit message(s)
• Commits are squashed into relevant entities - avoid a lot of minimal dev time commits in the PR
• Contribution guidelines followed
• Ghaf documentation updated with the commit - https://tiiuae.github.io/ghaf/
• PR linked to architecture documentation and requirement(s) (ticket id)
• Test procedure described (or includes tests). Select one or more:
  • Tested on Lenovo X1 x86_64
  • Tested on Jetson Orin NX or AGX aarch64
  • Tested on Polarfire riscv64
• Author has run make-checks and it passes
• All automatic Github Action checks pass - see actions
• Author has added reviewers and removed PR draft status
• Change requires full re-installation
• Change can be updated with nixos-rebuild ... switch

Instructions for Testing

Check that image works as expected after manual decompression or check that flash script succeeds.

[milva-unikie]

Tested on NUC (generic-x86_64-debug)

• The size of the compressed image is 3,1 GB
• When uncompressed the size is 13,3 GB
• I built, flashed and booted, works great

This change will require changes to test-automation. Those will need to be prepared before this pr is merged. They are under work now.

I will add the bug-label here until the test-automation has been fixed to support this pr. We don't really have an accurate label for this situation.

[milva-unikie]

Test-automation has been checked and it works with this pr. This can now be merged.