Only install mockgen if it exists
Only install mockgen if it exists

Signed-off-by: Dave Goddard <bfcdf3e6ca6cef45543bfbb57509c92aec9a39fb@goddard.id.au>

cloudfoundry/buildpacks-ci,cloudfoundry/buildpacks-ci,cloudfoundry/buildpacks-ci,cloudfoundry/buildpacks-ci,cloudfoundry/buildpacks-ci