Blessing `rmake.rs` tests silently does nothing

[Zalathar]

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/.

[Zalathar]

cc @jieyouxu @Kobzol
@rustbot label +A-run-make T-bootstrap