cache: allow skipping capability checks

[Xuanwo]

Remote cache capability checks read and write the shared .sccache_check object, which can trigger mutation rate limits when many sccache servers start concurrently.

Add SCCACHE_SKIP_CACHE_CHECK to bypass remote capability probes and use each backend's configured rw_mode, including multi-level caches.

Fixes #2132.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 90.51724% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.70%. Comparing base (bf75727) to head (93b180c).

Files with missing lines	Patch %	Lines
src/cache/cache.rs	87.83%	9 Missing ⚠️
src/cache/multilevel.rs	75.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2822      +/-   ##
==========================================
+ Coverage   73.64%   73.70%   +0.05%     
==========================================
  Files          72       72              
  Lines       37814    37915     +101     
==========================================
+ Hits        27849    27946      +97     
- Misses       9965     9969       +4     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.