build: only run add-metainfo-releases if we have it
build: only run add-metainfo-releases if we have it

If we're building a dist from a tarball release, the script won't be
present, but that's OK: we already have the release info in the .xml
file in that case.

martinpitt/cockpit,cockpit-project/cockpit,mvollmer/cockpit,cockpit-project/cockpit,martinpitt/cockpit,cockpit-project/cockpit,cockpit-project/cockpit,mvollmer/cockpit,mvollmer/cockpit,martinpitt/cockpit,mvollmer/cockpit,cockpit-project/cockpit,martinpitt/cockpit,mvollmer/cockpit,martinpitt/cockpit