Add (unstable) documentation for --env command line option

Author: GuillaumeGomez

src/doc/unstable-book/src/compiler-flags/env.md

@@ -0,0 +1,21 @@
+# `env`
+
+The tracking issue for this feature is: [#118372](https://github.com/rust-lang/rust/issues/118372).
+
+------------------------
+
+This option flag allows to specify environment variables value at compile time to be
+used by `env!` and `option_env!` macros.
+
+When retrieving and environment variable value, the one specified by `--env` will take
+precedence. For example if you want have `PATH=a` in your environment and pass:
+
+```bash
+rustc --env PATH=env
+```
+
+Then you will have:
+
+```rust
+assert_eq!(env!("PATH"), "env");
+```