rustfmt breaks when `rustc` is set in `config.toml`

[KittyBorgX]

Steps for reproduction:

• Set cargo and rustc under the build section in config.toml
  Here's mine:

[build]
rustc = "/Users/kitty/.rustup/toolchains/beta-2023-01-25-aarch64-apple-darwin/bin/rustc"
cargo = "/Users/kitty/.rustup/toolchains/beta-2023-01-25-aarch64-apple-darwin/bin/cargo"

• Run ./x.py fmt

I expected to see this happen: It formats the files

Instead, this happened: Here's the error:

➜  rust git:(master) ✗ ./x.py fmt  
Building bootstrap
    Finished dev [unoptimized] target(s) in 0.10s
skip untracked path library/build/ during rustfmt invocations
skip untracked path src/bootstrap/build/ during rustfmt invocations
Unrecognized option: 'unstable-features'
Unrecognized option: 'unstable-features'
Unrecognized option: 'unstable-features'
Unrecognized option: 'unstable-features'
Running `"/Users/kitty/.rustup/toolchains/beta-2023-01-25-aarch64-apple-darwin/bin/rustfmt" "--config-path" "/Users/kitty/projects/rust" "--edition" "2021" "--unstable-features" "--skip-children" "/Users/kitty/projects/rust/src/librustdoc/html/length_limit.rs" "/Users/kitty/projects/rust/src/librustdoc/fold.rs" "/Users/kitty/projects/rust/src/librustdoc/html/format.rs" "/Users/kitty/projects/rust/src/librustdoc/theme/tests.rs" "/Users/kitty/projects/rust/src/librustdoc/html/highlight/fixtures/sample.rs" "/Users/kitty/projects/rust/src/librustdoc/html/highlight/fixtures/union.rs" "/Users/kitty/projects/rust/src/librustdoc/docfs.rs" "/Users/kitty/projects/rust/src/librustdoc/visit_ast.rs"` failed.
If you're running `tidy`, try again with `--bless`. Or, if you just want to format code, run `./x.py fmt` instead.
Unrecognized option: 'unstable-features'
Build completed unsuccessfully in 0:00:02
Unrecognized option: 'unstable-features'
Unrecognized option: 'unstable-features'
Unrecognized option: 'unstable-features'                                                                                                                                                                                                                             
Unrecognized option: 'unstable-features'

Meta

rustc --version --verbose:

rustc 1.67.0 (fc594f156 2023-01-24)
binary: rustc
commit-hash: fc594f15669680fa70d255faec3ca3fb507c3405
commit-date: 2023-01-24
host: aarch64-apple-darwin
release: 1.67.0
LLVM version: 15.0.6

rustc +beta-2023-01-25-aarch64-apple-darwin --version --verbose:

rustc 1.68.0-beta.1 (efd27454a 2023-01-25)
binary: rustc
commit-hash: efd27454ae94386e11997b1158f60d3a79deeb7d
commit-date: 2023-01-25
host: aarch64-apple-darwin
release: 1.68.0-beta.1
LLVM version: 15.0.6

Comments:
unstable-features exists only in nightly rust since I am able to run it using .rustup/toolchains/nightly-aarch64-apple-darwin/bin/rustfmt. Setting rustc to stable or beta rustc breaks it.

I would like to take this up if it can be reproduced for others

[jyn514]

Duplicate of #81155