use stable nox for mingw (#512)

Author: davidhewitt

.github/workflows/ci.yml

@@ -400,8 +400,7 @@ jobs:
 
       - name: Install test dependencies
         shell: msys2 {0}
-        # TODO: use stable nox after the targeted commit is released
-        run: python -m pip install --upgrade pip 'nox @ git+https://github.com/wntrblm/nox.git@60acbbe5574e4458931d96ac4733b91d0db1b50d'
+        run: python -m pip install --upgrade pip nox
 
       - name: Create libpython symlink
         shell: msys2 {0}