cockpit: fix subprocess invocation in frozen binaries

[ThomasWaldmann]

When running as a Pyinstaller-made binary, sys.executable points to the borg binary itself. Invoking it with "-m borg" resulted in an incorrect command line (e.g., "borg -m borg ..."), which confused the argument parser in the subprocess.

This change checks sys.frozen to determine the correct invocation:

• If frozen: [sys.executable, ...args]
• If not frozen: [sys.executable, "-m", "borg", ...args]

Fixes #9277.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 75.92%. Comparing base (94d56e1) to head (054194f).
⚠️ Report is 11 commits behind head on master.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/borg/cockpit/runner.py	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9331      +/-   ##
==========================================
- Coverage   75.93%   75.92%   -0.02%     
==========================================
  Files          86       86              
  Lines       14780    14782       +2     
  Branches     2202     2203       +1     
==========================================
  Hits        11223    11223              
- Misses       2880     2882       +2     
  Partials      677      677              

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