Use version files instead of main branch when compiling packages

The following discussion from !3821 (merged) should be addressed:

• @kkloss started a discussion:

  suggestion (non-blocking): right now we’re compiling the latest version of gitaly and storing it by checksum. but the most compatible version would be the one defined in gitlab-org/gitlab>’s /GITALY_SERVER_VERSION file.

  WDYT about using the gitaly commit hash to both check the downloaded/compiled gitaly version and to only compile gitaly when we don’t have one in the package registry for that sha? Then we could also fall back to local compilation when the GITALY_SERVER_VERSION was updated but we don’t have a new binary yet.

  I’ll leave this as non-blocking because it’d probably be a major logic change.