Syncronize on unavailableRepositories before access the list

[laeubi]

Currently unavailableRepositories is not synced what can lead in some rare cases to strange behaviors.

This now uses the existing reporistoryLock to make sure only one thread is acessing the datastructure at a time.

[github-actions[bot]]

Test Results

       9 files  ±0         9 suites  ±0   35m 9s :stopwatch: -5s 2 177 tests ±0  2 173 :heavy_check_mark: ±0    4 :zzz: ±0  0 :x: ±0  6 621 runs  ±0  6 610 :heavy_check_mark: ±0  11 :zzz: ±0  0 :x: ±0 

Results for commit ff0ca594. ± Comparison against base commit e55ba98e.