git: Globally disable generation of server info for git-repack(1)

In 0d588e40 (git: Disable generation of server info in git-repack(1), 2022-03-11), we have started to disable generation of server info in git-repack(1) by passing the -n flag to all invocations. This was the best we could do because Git didn't provide any way to do this on a global level. We have since upstreamed a new repack.updateServerInfo config key though which allows us to do so.

The new config key was released with Git v2.36.0, so let's start injecting it into git-repack(1) processes. Note that we cannot yet get rid of the -n flag because the minimum required Git version is still at v2.33.1, which doesn't understand the new config. But by injecting the config via environment variables, Git will now also honor it in case git-repack(1) was spawned via any intermediate command.

Further note that we cannot really test this either because we don't yet have Git v2.36.0 as part of our CI infrastructure.

Closes #4123 (closed)