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_os.h

@@ -124,8 +124,7 @@ bool
 disk_cache_enabled(void);
 
 bool
-disk_cache_load_cache_index(void *mem_ctx, struct disk_cache *cache,
-                            char *path);
+disk_cache_load_cache_index(void *mem_ctx, struct disk_cache *cache);
 
 bool
 disk_cache_mmap_cache_index(void *mem_ctx, struct disk_cache *cache,