fix: Correctly remove bk dir when re-signing macOS Python 2.x executable

Fixes #2271

> PermissionError: [Errno 1] Operation not permitted:
'/Users/alex/src/virtualenv/v27/bin/bk'

Requires #2270
Refs #2233

docs/changelog/bugfix.2271.rst

@@ -0,0 +1,2 @@
+Fix ``PermissionError: [Errno 1] Operation not permitted`` when creating a
+Python 2.x virtualenv on macOS/arm64 - by ``moreati``

src/virtualenv/create/via_global_ref/builtin/cpython/mac_os.py

@@ -136,7 +136,7 @@ def fix_signature(self):
             bak_dir.mkdir(parents=True, exist_ok=True)
             subprocess.check_call(["cp", exe, bak_dir])
             subprocess.check_call(["mv", bak_dir / exe.name, exe])
-            bak_dir.unlink()
+            bak_dir.rmdir()
             cmd = ["codesign", "-s", "-", "--preserve-metadata=identifier,entitlements,flags,runtime", "-f", exe]
             logging.debug("Changing Signature: %s", cmd)
             subprocess.check_call(cmd)