fix: protect cache backend with locks

[aawsome]

closes rustic-rs/rustic#1875

[codecov]

Codecov Report

❌ Patch coverage is 93.75000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 53.1%. Comparing base (1030be4) to head (1d74ca8).

Files with missing lines	Patch %	Lines
crates/core/src/backend/cache.rs	93.7%	1 Missing ⚠️

Additional details and impacted files

Files with missing lines	Coverage Δ
crates/core/src/backend/cache.rs	60.2% <93.7%> (+2.6%)	⬆️

... and 22 files with indirect coverage changes

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