grafana: avoid copying go files in tree

Created by: keegancsmith

The grafana build script temporarily copies ./monitoring to ./docker-images/grafana/monitoring. ./monitoring contains go files. However, whenever go files change our dev script would want to rebuild the things that have changed. So we would get into this state where the grafana build script would keep adding and removing go files, and our dev script would keep deciding we need to rebuild grafana.

This commit does the build out of tree instead, to avoid the watch script seeing the go files come and go. An alternative implementation would be adding an ignore to the temporary directory, but that requires touching more places. Instead we adjust the hack used by grafana/build.sh.