Fix -release repository check in release.py

[j-rivero]

Fix the fail in the check for github repository, that now displays the error:

Error running command (git ls-remote -q --exit-code https://foo:foo@github.com/gazebo-release/gz-cmake-release).
stdout: 
stderr: remote: Support for password authentication was removed on August 13, 2021.
remote: Please see https://docs.github.com/en/get-started/getting-started-with-git/about-remote-repositories#cloning-with-https-urls for information on currently recommended modes of authentication.
fatal: Authentication failed for 'https://github.com/gazebo-release/gz-cmake-release/'

[scpeters]

works for me