Build git using symlinks to reduce size

!4506 (merged) changed how we build git and resulted in 100s of MB of size increase in the omnibus packages and cloud native docker images, which is causing builds to fail as the artifacts are now too large to be uploaded to GitLab.

This is due to git using hardlinks by default. Because in the omnibus build, and in the CNG images, there is a copy step (one in omnibus to package up the files, one in CNG to move files from a BUILD container to the final image). The hardlinks are turned into duplicates.

This change should bring the sizes back down by forcing git to use symlinks. We have seen that this is working for CNG and omnibus at least with: gitlab-org/build/CNG!1000 (merged) and omnibus-gitlab!6071 (merged)