Add support for extra rustc environment variables (#3510)

This adds support for injecting environment variables into rustc
invocations via build settings:

- `//rust/settings:extra_rustc_env` for the target configuration
- `//rust/settings:extra_exec_rustc_env` for the exec configuration

These settings can be set from the command line with flags like:

```
  --@rules_rust//rust/settings:extra_rustc_env=RUSTC_BOOTSTRAP=1
```

This mirrors how extra rustc flags are handled and enables build-wide
env-based feature control without editing individual rules. Useful for
enabling unstable features across many crates, or toggling behavior in
proc macros and build scripts.