ci: Allow ANGLE_TAG to be unset in -runner scripts

The android-runner scripts use `set -u`, which causes them to fail if
ANGLE_TAG is not defined. This issue went unnoticed because all current
Android jobs set ANGLE_TAG.

Update all -runner scripts to tolerate ANGLE_TAG being unset.

Signed-off-by: Valentine Burley <valentine.burley@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/35362>

.gitlab-ci/cuttlefish-runner.sh

@@ -9,7 +9,7 @@ set -uex
 section_start cuttlefish_setup "cuttlefish: setup"
 
 # Structured tagging check for angle
-if [ -n "$ANGLE_TAG" ]; then
+if [ -n "${ANGLE_TAG:-}" ]; then
     # Bail out if the ANGLE_TAG differs from what is offered in the system
     ci_tag_test_time_check "ANGLE_TAG"
 fi