legacy: use borg 1.x keys dir for v1 repos

[mr-raj12]

Description

Keyfile lookup always used the borg2 keys dir. On macOS that path (~/Library/Application Support/borg/keys) differs from the borg 1.x one (~/.config/borg/keys), so keyfiles of v1 repos were not found.

_keys_dir() now returns the borg 1.x keys dir when repository.version == 1, the same way security._security_dir picks the security dir. Both keyfile lookup paths use it. New key creation stays on the borg2 dir.

Part of #9556.

Checklist

• PR is against master
• New code has tests and docs where appropriate
• Tests pass (relevant crypto/key and fs subsets)
• Commit messages are clean and reference related issues

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.12%. Comparing base (3b3a647) to head (46642da).
⚠️ Report is 5 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9831      +/-   ##
==========================================
- Coverage   85.19%   85.12%   -0.07%     
==========================================
  Files          92       93       +1     
  Lines       15303    15345      +42     
  Branches     2300     2307       +7     
==========================================
+ Hits        13037    13063      +26     
- Misses       1578     1587       +9     
- Partials      688      695       +7     

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