Pip install suddenly requires --user flag on Windows

As of this morning (29 July 2020) running pip install without the --user flag fails on Windows with

 ERROR: Could not install packages due to an EnvironmentError: [WinError 5] Access is denied: 'C:\\Users\\RUNNER~1\\AppData\\Local\\Temp\\pip-uninstall-z6z1a6y6\\pip.exe'
Consider using the `--user` option or check the permissions.

You can find the failing workflow here and a working workflow from yesterday evening (28 July 2020) here.

Actually, this has nothing to do with GitHub Actions. This was causes by the release of pip==20.2 last night. I’ve opened a bug report here.