x.py check doesn't take previous x.py build into account

[varkor]

After successfully running x.py build, we know x.py check is also going to succeed. It shouldn't then try re-checking rustc. (This should probably take into account the build on an incremental basis.)

Issue on cargo: rust-lang/cargo#3501

[Mark-Simulacrum]

I'm going to close this since I don't think this is something that's feasible or wanted to fix on rustbuild's side. Of course, improvements to Cargo in this area are much appreciated. Thanks!