Set environment variables in rust-toolchain.toml #2883

Describe the problem you are trying to solve

I'm managing a sort of meta-build system that manages packages built in many different languages. It aims to delegate to the idiomatic build tools for each language as much as possible rather than re-implement them, which means it relies heavily on being able to express all necessary configuration through idiomatic configuration files. Unfortunately, there are some configuration options that cannot (currently) be expressed easily in configuration options, and instead have to take the form of environment variables. These are unfortunate because while the meta-build system knows to set those environment variables before it delegates to Cargo, it breaks the ability for human users to "just run cargo", which also tends to break IDE integration provided by rust-analyzer and the like, as they also just execute cargo directly and expect that to work.

Some of these can be dealt with by using the new [env] section in .cargo/config.toml, but since those are only set after Cargo is invoked, it doesn't allow setting environment variables that might affect Cargo itself or Rustup toolchains. While it would be possible to set these using something like direnv, that is an additional tool that users would have to have set up and working, and if they don't things would fail in surprising and hard-to-diagnose ways.

Concretely, I want to change PATH and CARGO_HOME:

• I want to change PATH so that tools provided by the meta-build system (and idiomatic dependencies declared therein) are made available to custom toolchains. The meta-build system injects custom toolchains that ensure that the local build uses the rustc/cargo vended through the meta-build system rather than whatever the user happens to have installed locally, and those toolchains in turn rely on plenty of commands being available on PATH to do their job. They coincidentally also rely on other custom environment variables, which I would also set in a hypothetical rust-toolchain.toml [env] section.
• I want to change CARGO_HOME both to control where Cargo places its various cache files so that they end up in a place that the meta-build system can track (and clean) and where they don't clutter the users' normal Cargo setup, and to avoid accidentally picking up user-specific configuration from ~/.cargo/config.toml which might make local build artifacts differ in unpredictable ways leading to hard-to-diagnose issues. CARGO_HOME needs to be set before cargo is invoked, as it affects where Cargo looks for configuration files in the first place, and thus can't be set in Cargo's [env].

Describe the solution you'd like

I propose we add an [env] section to rust-toolchain.toml similar to the one supported by Cargo which allows setting environment variables inside Rustup itself before dispatching to the binary in the toolchain.

Open questions

• The ability to set environment variables is fairly powerful, and could be abused by setting HOME or PATH in such a way that program execution does "bad things". In all likelihood, this means [env] should be subject to the same "trust decision" as is being proposed in Environment variable to disable rust-toolchain.toml #2793.
• Cargo's [env] syntax allows setting config-relative paths rather than absolute ones with a relative = true option. Should we support the same? I don't know of such paths existing in Rustup currently.
• Cargo's [env] does not (currently) provide a mechanism for augmenting an environment variable, such as by allowing PATH to be set to some value that includes $PATH. Should it? My instinct is to follow the design of Cargo's [env] section closely, and only add this if Cargo adds support for the same.