Auto merge of #9531 - 5225225:cargo-doc-open-fix, r=ehuss

Fix `BorrowMutError` when calling `cargo doc --open`

~~I'm not sure why the existing test suite didn't catch this, it definitely calls `cargo doc --open`.~~

I had

```toml
[doc.extern-map]
std = "local"
```

in my `.cargo/config.toml`. Will write a test case that sets that and then tries to run `cargo doc --open`.

Closes #9530

diff --cc tests/testsuite/doc.rs
Simple merge