gitlab-ci: Use full clone for trigger_snap_builds job

The push to GitHub fails because the shallow clone is missing objects
that the remote references. Disable shallow cloning for this job by
setting GIT_DEPTH to 0

Signed-off-by: Simon Glass <simon.glass@canonical.com>

.gitlab-ci.yml

@@ -968,6 +968,8 @@ efi-aarch64:
 
 trigger_snap_builds:
   stage: release
+  variables:
+    GIT_DEPTH: 0  # Need full history for GitHub push
   rules:
     - if: $CI_COMMIT_BRANCH == "master"
   before_script: