Blessing `rmake.rs` tests silently does nothing

While working on #129037, I noticed that running an `rmake.rs` test with `--bless` appears to succeed, but doesn't actually update the snapshot file(s).

This seems to be a side-effect of #126097. The test does successfully bless the temporary copy of the snapshot file in `build/`, but that has no effect on the original snapshot file in `tests/run-make/`.
