fix: add mutex to LocalRepositoryCache to prevent concurrent map panic

[jun-mercury]

Summary

LocalRepositoryCache.CacheDirCache is a plain map[string]string accessed concurrently by Fetch (write) and GetTarArchive (read) when multiple matrix jobs resolve --local-repository actions in parallel. This causes an intermittent panic:

fatal error: concurrent map read and map write

goroutine 313114 [running]:
LocalRepositoryCache.GetTarArchive(...)
    local_repository_cache.go:47

This is the same class of bug as #6028 (fixed in #6029 for GoGitActionCache), but in the --local-repository code path. The GoGitActionCache fix added per-repo mutexes to serialize git operations — this PR applies the same pattern to LocalRepositoryCache.

Fix

Add a sync.RWMutex to serialize access to CacheDirCache:

• Fetch(): write lock when updating the cache
• GetTarArchive(): read lock when looking up cached paths

LocalRepositories is set at initialization and only read after, so it does not need synchronization.

Reproduction

1. Create a workflow calling a reusable workflow with a matrix (2+ entries)
2. Each matrix job references the same external action via --local-repository
3. Run with act using containerized runners
4. Failure is intermittent — depends on goroutine scheduling

Crash observed in CI: https://github.com/MercuryTechnologies/mercury-web-backend/actions/runs/23767992978/job/69252242640

Context

Hi @cplee — this is a companion to #6029, which we submitted a few weeks ago for the same race condition in GoGitActionCache. We've been running act with the #6029 patch in our CI for a couple of weeks now (hundreds of runs across multiple environments) and it's been stable. This new crash surfaced because LocalRepositoryCache has the same unprotected map pattern but in a different code path that we didn't catch initially.

We'd appreciate a review when you get a chance — both this and #6029 are small, targeted fixes. Happy to address any feedback.

Related: #6028 (issue), #6029 (GoGitActionCache fix)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.90%. Comparing base (bd4bc99) to head (6ea89d9).
⚠️ Report is 46 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6055      +/-   ##
==========================================
+ Coverage   74.65%   76.90%   +2.24%     
==========================================
  Files          73       74       +1     
  Lines       11139     9325    -1814     
==========================================
- Hits         8316     7171    -1145     
+ Misses       2186     1510     -676     
- Partials      637      644       +7     

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[HAHGX]

@jun-mercury FYI