Use token for git push

[fingolfin]

When the release script does a git push (twice, in fact), then this currently requires a user password, leading to password prompts if you don't use SSH or a credential store.

Just use the access token we have anyway to avoid that!
Basically, just push to https://$USER:[email protected]/$REPO, where we just have to extract $USER from $REPO, which is of the form USERNAME/PKGNAME.