ci: fix OCI registry authentication for buildcache push

.github/workflows/buildcache.yml

@@ -39,7 +39,7 @@ jobs:
 
       - name: Login to OCI registry
         shell: spack-bash {0}
-        run: spack oci login ghcr.io --username ${{ github.actor }} --password-stdin <<< ${{ secrets.GITHUB_TOKEN }}
+        run: spack -e fairmq mirror set --push --oci-username ${{ github.actor }} --oci-password-variable GITHUB_TOKEN local-buildcache
 
       - name: Push to buildcache
         if: ${{ !cancelled() }}

test/ci/spack.yaml

@@ -1,10 +1,6 @@
 spack:
   specs:
-    - "fairlogger@2.2.0 ^fmt@:11"
-    - "boost@1.66: +container +program_options +filesystem +date_time +regex"
-    - "libzmq@4.1.4:"
-    - ninja
-    - "cmake@3.15:"
+    - zlib-ng
 
   config:
     install_tree: