bootstrap ignores `{CC,CFLAGS}_$TARGET` env vars when target name contains a period

Summary

What the title says.

Command used

$ env CC_thumbv8m.main_none_eabihf=false ./x build --target thumbv8m.main-none-eabihf --stage 1 library/compiler-builtins && echo OK
(..)
OK

Expected behaviour

The step of building C code should fail because the C cross compiler is overridden to the false command.

Actual behaviour

The env var is ignored and the default C compiler is used.

Bootstrap configuration (bootstrap.toml)

profile = 'dist'

[llvm]
download-ci-llvm = true

Operating system

Ubuntu 22.04.5 LTS

HEAD

32fe406b5e71afbb0d8b95280e50e67d1549224c

Additional context

The override works when the target name does not have a period in the name. For example, this command fails as expected:

$ env CC_thumbv7em_none_eabihf=false ./x build --target thumbv7em-none-eabihf --stage 1 library/compiler-builtins
(..)
error occurred: Command "false" "-O3" (..)

The cc (v1.2.16) crate also respects the env var when used outside the bootstrap environment.

$ cargo new --lib repro
$ cd repro
$ tail -n2 Cargo.toml
[build-dependencies]
cc = "=1.2.16"

$ cat build.rs
fn main() {
    dbg!(cc::Build::new().get_compiler());
    panic!() // so we can inspect the build script output
}

$ env CC_thumbv8m.main_none_eabihf=false cargo b --target thumbv8m.main-none-eabihf
(..)
  [build.rs:2:5] cc::Build::new().get_compiler() = Tool {
      path: "false",

So possibly bootstrap is modifying the environment that it passes to the cc crate when building the compiler-builtins crate.

---

In the case of the CC_$TARGET env var, the target.$TARGET.cc setting in bootstrap.toml can be used. I don't know if there's an easy workaround for CFLAGS_$TARGET.

Start by reproducing the issue with the provided bootstrap command and compare it with the working thumbv7em target case. Trace how bootstrap passes environment variables to cc while building compiler-builtins, using the bootstrap.toml target setting and the cc crate behavior as references. Done means CC_$TARGET and CFLAGS_$TARGET overrides still work when the target name contains a period.