bootstrap: fingerprinting is broken somehow

[matthiaskrgr]

I noticed that 2 subsequent x.py check runs would recheck a bunch of tools even with no code change in between :/

repro:
inside git repo, run

RUSTFLAGS="" RUSTC_WRAPPER=""  ./x.py check
RUSTFLAGS="" RUSTC_WRAPPER=""  ./x.py check

the second run will not finish immediately but recheck some stuff for some reason.

[matthiaskrgr]

Since we just updated bootstrap, this might as well be a cargo bug of some sort.

[onur-ozkan]

#117850 fixes this.