Fix read from null pointer in CacheHash.hit

It occured when the manifest file was manually edited to include an extra
file. Now it will simply copy the file name in the manifest file

lib/std/cache_hash.zig

@@ -186,6 +186,10 @@ pub const CacheHash = struct {
                 return error.InvalidFormat;
             }
 
+            if (cache_hash_file.path == null) {
+                cache_hash_file.path = try mem.dupe(self.alloc, u8, file_path);
+            }
+
             const this_file = fs.cwd().openFile(cache_hash_file.path.?, .{ .read = true }) catch {
                 return error.CacheUnavailable;
             };