ci: fix envvar default value

With 047bb6b8 on !35740, when GIT_STRATEGY is not defined, the scripts can
fail where we use `set -u` to raise an error when unset variables.

Signed-off-by: Sergi Blanch Torne <sergi.blanch.torne@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/40759>

.gitlab-ci/piglit/piglit-traces.sh

@@ -135,7 +135,7 @@ replay_s3_upload_images() {
 
 SANITY_MESA_VERSION_CMD="$SANITY_MESA_VERSION_CMD | tee /tmp/version.txt | grep \"Mesa $MESA_VERSION\(\s\|$\)\""
 
-if [ "$GIT_STRATEGY" = none ]; then
+if [ "${GIT_STRATEGY:-}" = none ]; then
   cd $RESULTS_DIR && rm -rf ..?* .[!.]* *
 fi
 cd /piglit

.gitlab-ci/setup-test-env.sh

@@ -130,7 +130,7 @@ export -f _error_msg
 
 if [ -z "${RESULTS_DIR:-}" ]; then
 	export RESULTS_DIR="${PWD%/}/results"
-	if [ "$GIT_STRATEGY" = none ]; then
+	if [ "${GIT_STRATEGY:-}" = none ]; then
 		rm -rf "${RESULTS_DIR}"
 	fi
 fi

.gitlab-ci/test/gitlab-ci-inc.yml

@@ -149,7 +149,7 @@
 
 .deqp-test:
   script:
-    - if [ "$GIT_STRATEGY" = none ]; then rm -rf results; fi  # Clear out old results if the docker container was cached
+    - if [ "${GIT_STRATEGY:-}" = none ]; then rm -rf results; fi  # Clear out old results if the docker container was cached
     - ./install/deqp-runner.sh
   artifacts:
     exclude:
@@ -175,6 +175,6 @@
     # setup).
     - section_start artifacts_download "Downloading artifacts from s3"
     # Note: Build dir (and thus install) may be dirty due to GIT_STRATEGY
-    - if [ "$GIT_STRATEGY" = none ]; then rm -rf install; fi
+    - if [ "${GIT_STRATEGY:-}" = none ]; then rm -rf install; fi
     - (set -x; curl -L --retry 4 -f --retry-all-errors --retry-delay 60 ${FDO_HTTP_CACHE_URI:-}https://${PIPELINE_ARTIFACTS_BASE}/${S3_ARTIFACT_NAME}.tar.zst | tar --zstd -x)
     - section_end artifacts_download