feat: Add arm64 build and test support

[raharper]

• Update github build workflow to build on 24.04 x86_64 and arm64
• Update artifacts to be named atomfs-$GOOS-$GOARCH:
  • atomfs-linux-amd64
  • atomfs-linux-arm64
• Update tests to skip building pre-erofs image if not on x86_64 arch since we don't have an arm64 build of the pre-erofs stacker
• Updated comment in verity.go to drop default size for x86; it's just the default size for verity struct

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 37.21%. Comparing base (29b8859) to head (2665815).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #53      +/-   ##
==========================================
- Coverage   39.47%   37.21%   -2.26%     
==========================================
  Files          24       24              
  Lines        1758     2257     +499     
==========================================
+ Hits          694      840     +146     
- Misses        892     1248     +356     
+ Partials      172      169       -3     

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[raharper]

If we land; we can update the required Actions and replace the "build 22.04" and "build 24.04" with the two build-linux-$ARCH builds.