gitlab: Propagate stack name to downstream build jobs (#29019)

It will be useful for metrics gathering and possibly debugging to
have this environment variable available in the runner pods that
do the actual rebuilds.

lib/spack/spack/ci.py

@@ -1114,6 +1114,10 @@ def generate_gitlab_ci_yaml(env, print_summary, output_file,
         if pr_mirror_url:
             output_object['variables']['SPACK_PR_MIRROR_URL'] = pr_mirror_url
 
+        spack_stack_name = os.environ.get('SPACK_CI_STACK_NAME', None)
+        if spack_stack_name:
+            output_object['variables']['SPACK_CI_STACK_NAME'] = spack_stack_name
+
         sorted_output = {}
         for output_key, output_value in sorted(output_object.items()):
             sorted_output[output_key] = output_value