Fix `rustdoc --version` when used with download-rustc
Previously, rustdoc would unconditionally report the version that *rustc* was compiled with. That showed things like `nightly-2022-10-30`, which wasn't right, since this was a `dev` build compiled from source.
Fix it by changing `rustc_driver::version` to a macro expanded at invocation time.
cc https://github.com/rust-lang/rust/issues/103206#issuecomment-1284123084
70c04a2fd1