ci: improve buildcache workflow

- rename mirror to ghcr-buildcache
- find system compiler before building gcc
- separate update-index job to avoid race condition
- always attempt push even on partial failure

test/ci/spack.yaml

@@ -8,6 +8,6 @@ spack:
       padded_length: 128
 
   mirrors:
-    local-buildcache:
+    ghcr-buildcache:
       url: oci://ghcr.io/fairrootgroup/fairmq-spack-buildcache
       signed: false