rustdoc: add `--test-builder-wrapper` arg to support wrappers such as RUSTC_WRAPPER when building doctests
Currently, `rustdoc` builds test crates with `rustc` directly instead of using [`RUSTC_WRAPPER`](https://doc.rust-lang.org/cargo/reference/config.html#buildrustc-wrapper) (if any is set).
This causes build issues in build systems that use `cargo` but tweak linking flags by setting the `RUSTC_WRAPPER` environment variable.
This change is not meant to be final--it's only a minimal proof of concept.
Please advise on the best way to proceed.
Open questions:
- [x] Does supporting the `rustc` wrappers make sense?
- yes, `cargo-miri` for example needs a "hack" to workaround the issue
- [X] What environment variable(s) should be read for the rustc wrapper? Should `rustdoc` [use the same names as `cargo`](https://doc.rust-lang.org/cargo/reference/config.html#buildrustc-wrapper)?
- None, since `rustdoc` takes arguments
- [X] What name should be used for a `rustdoc` CLI option?
- `--test-builder-wrapper`
- [X] Should a separate workspace wrapper (like `RUSTC_WORKSPACE_WRAPPER`) be supported?
- `--test-builder-wrapper` can be passed multiple times to get multiple wrappers passed
- [X] How/where should this be documented? It's not obvious to all users that `cargo doc` actually causes `rustdoc` to compile tests with rust
- Added doc to `src/doc/rustdoc/src/command-line-arguments.md` per `@GuillaumeGomez`
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