Use remote git URL for forks
I wanted to review !916 (merged) from Janek but the resulting Instal Stackspin job fails with unable to clone 'https://open.greenhost.net/stackspin/stackspin': couldn't find remote ref "refs/heads/ipv6"
because Janek has this ipv6
branch on his fork.
So we need to make sure to install the remote URL/branch from the MR.
See Job #187202 for details.