Replace virtualenv with Python's built-in venv (#30377)

* Replace virtualenv with Python's built-in venv.

* Apply Delan's suggestions and make a couple small fixes

- Fix a tidy warning about directories that don't exist
- Use shutil instead of the redundant get_exec_path
- Miscellaneous cleanups

* Fix typo in environment variable

* fix bug where pip still tries to the wrong site-packages

---------

Co-authored-by: Martin Robinson <mrobinson@igalia.com>
Co-authored-by: Delan Azabani <dazabani@igalia.com>

.github/workflows/mac-wpt.yml

@@ -43,7 +43,7 @@ jobs:
       - name: Prep test environment
         run: |
           gtar -xzf target.tar.gz
-          python3 -m pip install --upgrade pip virtualenv
+          python3 -m pip install --upgrade pip
           python3 ./mach bootstrap
       - name: Smoketest
         run: python3 ./mach smoketest --${{ inputs.profile }}