util/disk_cache: make MESA_DISK_CACHE_READ_ONLY_FOZ_DBS a relative path

Rather than passing in full paths this changes things so that we can
just pass in filenames relative to the current cache directory.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/9279>

src/util/disk_cache.c

@@ -112,7 +112,7 @@ disk_cache_create(const char *gpu_name, const char *driver_id,
       goto path_fail;
 
    if (env_var_as_boolean("MESA_DISK_CACHE_SINGLE_FILE", false)) {
-      if (!disk_cache_load_cache_index(local, cache, path))
+      if (!disk_cache_load_cache_index(local, cache))
          goto path_fail;
    }