conftest: fix rmtree cleanup crash on Linux (os.lchflags does not exist)

[ThomasWaldmann]

The archiver fixture cleans up with shutil.rmtree(..., onerror=maybe_clear_flags_and_retry). That handler calls os.lchflags(path, 0) when has_lchflags is true, and retries the removal.

But has_lchflags is defined as:

has_lchflags = hasattr(os, "lchflags") or sys.platform.startswith("linux")

so it is also true on Linux, where flags are cleared via ioctl and os.lchflags does not exist. The resulting AttributeError is not an OSError, so it escapes the surrounding try/except OSError and turns teardown into an ERROR - the handler that exists to recover from a failed removal is itself broken on Linux.

Use borg's cross-platform platform.set_flags instead, which is the intended API and works on both BSD and Linux.

Split out of #9602 - the bug is independent of that PR and is present in master today.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.05%. Comparing base (91b030e) to head (54b1b3d).
⚠️ Report is 1 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #10124   +/-   ##
=======================================
  Coverage   87.05%   87.05%           
=======================================
  Files         101      101           
  Lines       17848    17849    +1     
  Branches     2705     2705           
=======================================
+ Hits        15537    15539    +2     
+ Misses       1609     1608    -1     
  Partials      702      702           

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