rustbuild warning

[est31]

When you compile rust you get:

warning: value assigned to `stage` is never read, #[warn(unused_assignments)] on by default
   --> src/bootstrap/flags.rs:277:17
    |
277 |                 stage = Some(1);
    |                 ^^^^^

Regression of #38072

cc @nikomatsakis

Maybe rustbuild should be compiled with #[forbid(warnings)] to prevent errors like these in the future?

[est31]

cc @alexcrichton

[alexcrichton]

Yeah when we fix this let's tag rustbuild with #![deny(warnings)]