rustbuild: `x.py doc -i --stage 1 src/libstd` builds the compiler twice

[QuietMisdreavus]

This seems to be new? I think in the past it worked without building the stage1 compiler. (At least, without the --incremental flag.) My current branch is based on master at 4122885.

Full build log with --verbose: https://gist.github.com/QuietMisdreavus/5d000903b7cc4f1aae655f7c9b3073b8

cc @Mark-Simulacrum

[kennytm]

(Hint: You could use ./x.py doc --stage 1 src/lib --dry-run -v to dump the build tree without actually building anything.)

I think this is old, because until #51302 we require a stage2 sysroot for anything rustdoc related. Perhaps need to update these lines:

rust/src/bootstrap/builder.rs

Lines 802 to 807 in c131bdc

	if cmd != "build" && cmd != "check" && want_rustdoc {
	cargo.env(
	"RUSTDOC_LIBDIR",
	self.rustc_libdir(self.compiler(2, self.config.build)),
	);
	}

[Mark-Simulacrum]

Yes, those lines look like the problem. We probably just need to change the compiler argument to the compiler passed to the cargo function