Rerun build script on RUSTC_BOOTSTRAP change

[dtolnay]

Equivalent of dtolnay/thiserror#269.

Example failure mode without this PR:

pub fn repro(e: &anyhow::Error) -> &std::backtrace::Backtrace {
    e.backtrace()
}

$ cargo +stable check  #(expected failure)
   Compiling anyhow v1.0.76
    Checking repro v0.0.0
error[E0599]: no method named `backtrace` found for reference `&anyhow::Error` in the current scope
 --> src/lib.rs:2:7
  |
2 |     e.backtrace()
  |       ^^^^^^^^^ method not found in `&Error`

For more information about this error, try `rustc --explain E0599`.
error: could not compile `repro` due to previous error

$ RUSTC_BOOTSTRAP=1 cargo +stable check  #(should work, but doesn't)
    Checking repro v0.0.0
error[E0599]: no method named `backtrace` found for reference `&anyhow::Error` in the current scope
 --> src/lib.rs:2:7
  |
2 |     e.backtrace()
  |       ^^^^^^^^^ method not found in `&Error`

For more information about this error, try `rustc --explain E0599`.
error: could not compile `repro` due to previous error

$ cargo +stable clean
     Removed 30 files, 9.9MiB total
$ RUSTC_BOOTSTRAP=1 cargo +stable check
   Compiling anyhow v1.0.76
    Checking repro v0.0.0
    Finished dev [unoptimized + debuginfo] target(s) in 0.59s

After this PR: RUSTC_BOOTSTRAP=1 cargo +stable check works the first time.