Fixes cleanup of append-only test tempfiles on macOS/BSD

The `test_extract_restores_append_flag` test leaves append-only
tempfiles around on macOS and FreeBSD that cannot be removed cleanly,
this was previously just ignored by the cleanup func but those files
occasionally caused lots of warning output on subsequent test runs.
Fixed by attempting to clear flags and retry whenever the cleanup
function fails.

src/borg/conftest.py

@@ -133,7 +133,21 @@ def archiver(tmp_path, set_env_variables):
     os.chdir(archiver.tmpdir)
     yield archiver
     os.chdir(old_wd)
-    shutil.rmtree(archiver.tmpdir, ignore_errors=True)  # clean up
+
+    def maybe_clear_flags_and_retry(func, path, _exc_info):
+        if has_lchflags:
+            # Clear any BSD flags (e.g. UF_APPEND) that may have prevented removal, then retry once.
+            try:
+                os.lchflags(path, 0)
+                func(path)
+            except OSError:
+                pass
+        else:
+            # Do nothing (equivalent to `ignore_errors=True`) if flags aren't supported on this platform.
+            pass
+
+    # Clean up archiver temp files
+    shutil.rmtree(archiver.tmpdir, onerror=maybe_clear_flags_and_retry)
 
 
 @pytest.fixture()