fix cache hash regression

fixes "warning: unexpected seek failure"

fix regression introduced by 27e31f0475

the fd should be closed only if returning with an error

src/cache_hash.cpp

@@ -437,7 +437,9 @@ Error cache_hit(CacheHash *ch, Buf *out_digest) {
                 return ErrorCacheUnavailable;
             }
         }
-        os_file_close(ch->manifest_file);
+        if (return_code != ErrorNone) {
+            os_file_close(ch->manifest_file);
+        }
         return return_code;
     }
     // Cache Hit