Postgres-exporter fails to build due to absolute path link

There was a problem building the fuse-postges-exporter image because the postgres_exporter source tree contains a symlink to an absolute path that doesn't exist. The tools/src link points to /home/will/src/go/src/github.com/wrouesnel/postgres_exporter/tools/vendor, as can be seen here:

https://github.com/jboss-fuse/postgres_exporter/blob/0.4.x.redhat-7-x/tools/src

This symlink has existed for a while, but recently Cachito, part of the OSBS tooling, has started complaining about links to locations outside the repository. 

This was fixed upstream a while ago: https://github.com/wrouesnel/postgres_exporter/commit/03457ae745be26e3bb833f21e91b5ed3e467d98b