In order to make builds more reproducible SOURCE_DATE_EPOCH was set to
the time of the last commit that touched the template. Since trying to
reproduce a build from a different revision is futile (the most obvious
reason is that the source-revisions property includes the HEAD commit
hash) and looking up the commit in question can take several seconds,
stop wasting time an just use HEAD.
Closes: #12314 [via git-merge-pr]
a4a229cf64