DM-54728: Use new has_repo_config API to check if the root is a butler repo

[timj]

This checks for the butler.yaml as well as directory existence and prevents some edge cases where the directory is still there but files have been cleared out.

Checklist

• ran Jenkins
• ran and inspected package-docs build
• added a release note for user-visible changes to doc/changes

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 88.72%. Comparing base (cc674f9) to head (a82f0e3).
⚠️ Report is 2 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
python/lsst/pipe/base/simple_pipeline_executor.py	0.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #560      +/-   ##
==========================================
- Coverage   88.77%   88.72%   -0.06%     
==========================================
  Files         159      159              
  Lines       22054    22053       -1     
  Branches     2623     2623              
==========================================
- Hits        19579    19567      -12     
- Misses       1839     1844       +5     
- Partials      636      642       +6     

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