ci: Make the FDO_UPSTREAM_REPO variable global

ci-fairy doesn't know how to to look at $CI_MERGE_REQUEST_PROJECT_PATH
right now, so if we don't manually set $FDO_UPSTREAM_REPO, ci-fairy will
(without verbose logging turned on) silently fall back on the source
repository project path for finding the branch point. This might fail if
the owner of the source repository hasn't updated the `master` branch of
their fork.

Related: https://gitlab.freedesktop.org/freedesktop/ci-templates/-/issues/32



Signed-off-by: Jonas Ådahl <jadahl@gmail.com>