Add an easy way to test `pip` from GitHub URL (#43414)

When PIP releases their new version they rarely release beta or
rc packages, but usually announce their intention of making a
release few days before in the pip mailing list - which we follow.

This PR allows to easily swith Airlfow builds to use `pip` from
GitHub URL, as well as switching the CI builds from `uv` to
`pip` by seting a `force pip` label.

This PR also fixes regexp checks for PIP version to use the
"URL" installation syntax for PIP when it is set as GitHub URL.