gitlab-ci.yml: tag 2.3.x pmb usage (MR 5676)

The otherwise standard master branch of pmbootstrap has many
breaking changes, and we want to keep stable on a stable pmb
branch

Signed-off-by: Robert Eckelmann <longnoserob@postmarketos.org>

.gitlab-ci.yml

@@ -9,6 +9,8 @@ after_script:
 stages:
   - lint
   - build
+variables:
+  PMBOOTSTRAP_TAG: 2.3.x
 
 # This defines the rules for when a pipeline should run.
 workflow: