Audit usages of `std::env::*_var` in the compiler

[compiler-errors]

We have a couple instances of env::*_var which are now unsafe in edition 2024. They're not necessarily obviously safe, so I've left them with a FIXME(edition_2024) -- we should fix them eventually.

rust/compiler/rustc_llvm/build.rs

Line 54 in 2836482

env::set_var(&key, env);

rust/compiler/rustc_feature/src/tests.rs

Line 6 in 2836482

std::env::set_var("RUSTC_BOOTSTRAP", env);

[bjorn3]

The rustc_llvm/build.rs one should be safe. We don't spawn any threads inside the build script.