--bypass-lock: also skip the cache lock

[ThomasWaldmann]

Fixes #7255.

--bypass-lock only skipped the repository lock, but the security checks run for every command still acquired the exclusive client-side cache lock via CacheConfig, although they only read the cache config. Thus, e.g. borg list --bypass-lock was blocked while a borg create using the same $HOME was running.

Now the bypass flag is passed down to assert_secure / CacheConfig, which then reads the cache config without locking. This is safe because the cache config is only ever written via SaveFile (atomic rename), so an unlocked reader always sees a complete file. When no lock is held, the relocated-repository handling only updates the security dir and does not write to the cache config.

Commands that really open the cache (e.g. borg info, borg config --cache) still acquire the cache lock, as they might write to the cache.

Not an issue on master: borg2 removed --bypass-lock and its cache has no lock.

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 90.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 82.13%. Comparing base (38b6dce) to head (d8e9d67).
⚠️ Report is 3 commits behind head on 1.4-maint.

Files with missing lines	Patch %	Lines
src/borg/cache.py	88.88%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##           1.4-maint   #10158      +/-   ##
=============================================
+ Coverage      82.11%   82.13%   +0.02%     
=============================================
  Files             38       38              
  Lines          11435    11437       +2     
  Branches        1802     1803       +1     
=============================================
+ Hits            9390     9394       +4     
+ Misses          1461     1459       -2     
  Partials         584      584              

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