ci: update SPACK_EXTRA_MIRROR to use branch ref instead of stack name

The University of Oregon is maintaining a local copy of our top-level
develop buildcache to speed up download times and reduce data egress charges.

Change the way this environment variable is used by replacing the stack name
with a ref name. This fixes a layout problem and allows us to maintain
separate develop vs. release buildcache copies at UO if the need arises.

Author: zackgalbreath

.ci/gitlab/configs/ci.yaml

@@ -22,7 +22,7 @@ ci:
         - k=$CI_GPG_KEY_ROOT/intermediate_ci_signing_key.gpg; [[ -r $k ]] && spack gpg trust -y $k
         - k=$CI_GPG_KEY_ROOT/spack_public_key.gpg; [[ -r $k ]] && spack gpg trust -y $k
       script::
-      - - if [ -n "$SPACK_EXTRA_MIRROR" ]; then spack mirror add local "${SPACK_EXTRA_MIRROR}/${SPACK_CI_STACK_NAME}"; fi
+      - - if [ -n "$SPACK_EXTRA_MIRROR" ]; then spack mirror add local "${SPACK_EXTRA_MIRROR}/${CI_COMMIT_REF_NAME}"; fi
         - spack config blame mirrors
         - spack spec "/${SPACK_JOB_SPEC_DAG_HASH}"
       - - spack --color=always ci rebuild -j ${SPACK_BUILD_JOBS} --tests --timeout 300 > >(tee ${SPACK_ARTIFACTS_ROOT}/user_data/pipeline_out.txt) 2> >(tee ${SPACK_ARTIFACTS_ROOT}/user_data/pipeline_err.txt >&2)