ci: define FDO_UPSTREAM_REPO

If this is not set, every fork will have to rebuild the image.
This is not what we want for at least 2 reasons:
- forks are not running the CI in the same environment than the
  upstream project
- this infers bandwidth costs as every fork has its own image
  that needs to be pulled by every runner.

Signed-off-by: Benjamin Tissoires <benjamin.tissoires@gmail.com>

ci/.gitlab-ci.yml

@@ -1,6 +1,7 @@
 variables:
   FDO_DISTRIBUTION_TAG: "2020-03-30"
   FDO_DISTRIBUTION_VERSION: buster
+  FDO_UPSTREAM_REPO: "virgl/virglrenderer"
   TEST_IMAGE: "$CI_REGISTRY_IMAGE/debian/$FDO_DISTRIBUTION_VERSION:$FDO_DISTRIBUTION_TAG"
 
 include: