Write repo caches in specfile specific files

In this way we'll never encounter weird errors, when bumping the
specfile version. There are other unrelated issues with repo caches,
but those can be resolved separately.

lib/spack/spack/repo.py

@@ -599,7 +599,9 @@ def _build_index(self, name: str, indexer: Indexer):
         """Determine which packages need an update, and update indexes."""
 
         # Filename of the provider index cache (we assume they're all json)
-        cache_filename = f"{name}/{self.namespace}-index.json"
+        cache_filename = (
+            f"{name}/{self.namespace}-specfile_v{spack.spec.SPECFILE_FORMAT_VERSION}-index.json"
+        )
 
         # Compute which packages needs to be updated in the cache
         index_mtime = self.cache.mtime(cache_filename)