Support per-stage RUSTFLAGS in rustbuild.

[eddyb]

Right now rustbuild invokes cargo which wants to rebuild the world if it sees a different RUSTFLAGS value, while my workflow involves changing RUSTFLAGS just to test stage1+ with -Z orbit.

It would be nice to have passing --stage1-rustflags "-Z orbit" to the python script (or the environment variable `RUSTFLAGS1="-Z orbit") just work.

Currently the Makefiles have something similar but it doesn't actually work for compiletest, just rustc.

[apaprocki]

Did rustbuild ever get support for this? This appears to also be breaking chroot builds that need to pass per-stage sysroot values.

[Mark-Simulacrum]

I'm going to unassign @alexcrichton from this; I think others should be able to implement this and as far as I know they don't have any particular concern with this specific bug.

[Mark-Simulacrum]

We support this via RUSTFLAG_STAGE_NOT_0 and RUSTFLAGS_STAGE_0 set in the environment today.

[eddyb]

@Mark-Simulacrum I wonder if we should switch to RUSTFLAGS_BOOTSTRAP and RUSTFLAGS_NOT_BOOTSTRAP.