test: disable artifacts cache with composefs

layers restored from a tarball won't be converted to composefs (for
now at least), so disable the cache when using composefs.

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>

contrib/cirrus/setup_environment.sh

@@ -54,6 +54,10 @@ if [[ "$CI_DESIRED_STORAGE" = "composefs" ]]; then
     if [[ "$PRIV_NAME" == "root" ]]; then
         CI_DESIRED_COMPOSEFS="+composefs"
 
+        # artifacts restored from a tarball won't be stored correctly
+        # in composefs, so we need to disable this feature for now.
+        export NO_TEST_CACHE=true
+
         # KLUDGE ALERT! Magic options needed for testing composefs.
         # This option was intended for passing one arg to --storage-opt
         # but we're hijacking it to pass an extra option+arg. And it