Prevent race condition in mixin installation

[kichristensen]

What does this change

This fixes an intermittent race condition that causes GitHub Actions builds to fail with the error:

error unmarshalling from mixins package cache.json: invalid character ']' after top-level value
Error: running "bin/porter mixin install docker-compose --version canary --url=" failed with exit code 1

The GetMixins() mage target previously installed mixins in parallel using errgroup.Group. This caused multiple porter mixin install commands to run concurrently, all attempting to read and write the same cache.json file without synchronization.

The savePackageInfo() function performs read-modify-write operations on cache.json without file locking. When multiple processes write simultaneously, the file becomes corrupted with invalid JSON.

Solution: Changed GetMixins() to install mixins sequentially instead of in parallel, ensuring only one process accesses cache.json at a time.

What issue does it fix

This addresses an intermittent CI/CD failure that occurs during the "Configure Agent" step in GitHub Actions workflows when running mage build.

Notes for the reviewer

• The trade-off is slightly longer build times during mixin installation (sequential vs parallel), but this only affects the initial build when mixins aren't cached
• The fix eliminates the race condition without requiring more complex file locking mechanisms
• Existing mixin installation logic remains unchanged - only the parallel execution was removed

Checklist

• Did you write tests?
  • No tests added. The race condition is intermittent and difficult to reproduce reliably in tests. The fix is straightforward (removing parallel execution).
• Did you write documentation?
  • No documentation changes needed. This is an internal build process fix with no user-facing changes.
• Did you change porter.yaml or a storage document record? Update the corresponding schema file.
  • N/A
• If this is your first pull request, please add your name to the bottom of our Contributors list. Thank you for making Porter better! 🙇‍♀️
  • N/A