Changes to `RUSTC_BOOTSTRAP` causes unwanted recompilations

I have a rather large project and I need to sometimes change `RUSTC_BOOTSTRAP` for some reasons. This causes `proc_macro2` to recompile, which in turn recompile a large portion of our dependency tree. I personally find it quite surprising that this happens as this behavior is not documented anywhere.

Cargo verbose log:
```
Dirty proc-macro2 v1.0.86: the env variable RUSTC_BOOTSTRAP changed
Compiling proc-macro2 v1.0.86

```

Offending source code:

https://github.com/dtolnay/proc-macro2/blob/aa9476b27932ae1b1b50bbfe0530b3b261fc1b72/build.rs#L131