Fixes #16427.

What does this PR try to resolve?

Build scripts, procedural macros, the linker and even the compiler itself¹ may output temporary files to std::env::temp_dir() during compilation. It is possible to request that programs use a different temporary directory by setting the TMPDIR (Unix) or TMP and TEMP (Windows) environment variables.

This PR makes Cargo set these environment variables to a temporary directory within the target directory.

This is useful when debugging, as it allows more easily inspecting the state the compiler was working with when something went wrong, especially if using -Csave-temps=yes, as relevant files can more easily be found in the project directory instead of being bundled in /tmp with everything else. It also makes it easier to clean up after compilation is finished (cargo clean would remove temporary files as well, instead of the user having to reboot for /tmp to be cleared).

Additionally, this is useful for preventing information leakage: /tmp is global and world readable², and this means that a different user on the same machine could figure out possibly sensitive details about the project you were building. This is even an issue on macOS where the temporary directory is scoped to the current user (getconf DARWIN_USER_TEMP_DIR), since you'd still be susceptible to Man-in-the-middle attacks by less privileged processes by the same user.

Finally, this makes build scripts and proc-macros more self-contained, which is useful when sandboxing them, such as that which I'm working on in cargo-sandbox.

Implementation notes

When running build scripts, the temporary directory we request is /path/to/target/{debug,release}/build/PKG-HASH/tmp (same as $OUT_DIR/../tmp). For rustc/rustdoc invocations, the directory we request is /path/to/target/tmp (same as $CARGO_TARGET_TMPDIR), see #9814 for breakage introduced in the past by trying to make it /path/to/target/{debug,release}/tmp.

I had a look at the standard library's implementation of std::env::temp_dir, it seems that only Hermit and Motor OS don't respect any environment variable when considering the temporary directory, so the test that I've added should work on all host platforms but those (I'd argue the onus is on those niche platforms to figure out a standard environment variable that they want to use for this, but I can submit bugs upstream if you want me to?)

How to test and review this PR?

1. Run a build script like:

   fn main() {
       fs::write(std::env::temp_dir().join("foo.txt"), b"Lorem ipsum").unwrap();
   }

   And verify that the file shows up in the project-local folder.
2. Do something similar in a proc-macro.
3. Run cargo rustc -- -Csave-temps=yes, and verify that the symbols.o file shows up in the project-local temporary directory.