ci_template: avoid exporting the mediafiles into build artifacts

ci_template: avoid exporting the mediafiles into build artifacts

Previously we where accidently exporting the whole repo of
gst-integration-testsuites which includes 350mb of raw media
files and made the artifacts storage explode through the roof
along with the CI bills fd.o had to pay for uploading and
redownloading the artifacts

To deal with this, we clean all the media files from the builddir
and when needed we copy them over from the cache in the docker image,
and then git fetch the repo.

Close #69