Previously, this would change the test output when RUSTUP_HOME was set:
```
---- [ui] ui/issues/issue-49851/compiler-builtins-error.rs stdout ----
diff of stderr:
1 error[E0463]: can't find crate for `core`
2 |
3 = note: the `thumbv7em-none-eabihf` target may not be installed
+ = help: consider downloading the target with `rustup target add thumbv7em-none-eabihf`
4
5 error: aborting due to previous error
6
```
Originally, I fixed it by explicitly unsetting RUSTUP_HOME in
compiletest. Then I realized that almost no one has RUSTUP_HOME set,
since rustup doesn't set it itself; although it does set RUST_RECURSION_COUNT
whenever it launches a proxy. Then it was pointed out that this runtime
check doesn't really make sense and it's fine to make it unconditional.
- Suggest using `rustup target add` if `RUSTUP_HOME` is set. I don't know if there's any precedent for doing this, but it seems harmless enough and it will be a big help.
- Add a note about `#![no_std]` if `std` is missing but not core
- On nightly, suggest using `cargo build -Z build-std` if `CARGO` is set
- Add a note that std may be unsupported if `std` is missing but not core
- Don't suggest `#![no_std]` when the load isn't injected by the
compiler
