Implement `--env` compiler flag (without `tracked_env` support) Part of https://github.com/rust-lang/rust/issues/80792. Implementation of https://github.com/rust-lang/compiler-team/issues/653. Not an implementation of https://github.com/rust-lang/rfcs/pull/2794. It adds the `--env` compiler flag option which allows to set environment values used by `env!` and `option_env!`. Important to note: When trying to retrieve an environment variable value, it will first look into the ones defined with `--env`, and if there isn't one, then only it will look into the environment variables. So if you use `--env PATH=a`, then `env!("PATH")` will return `"a"` and not the actual `PATH` value. As mentioned in the title, `tracked_env` support is not added here. I'll do it in a follow-up PR. r? rust-lang/compiler