Always build gitlab-metrics-exporter on boot

When first introducing this, we said we only want to build this "when necessary". I think that is a mistake; since we mount a local volume where the binary may still reside from a previous build, the make target never actually does anything for me. This means unless you manually drop the bin contents of the build directory, you will run with a stale version of the exporter.

I fixed this by cleaning first. Compilation takes only a second or so, so I don't think we need to be worried about performance here.