Auto merge of #121341 - GrigorenkoPV:bootstrap-rustup-cargo, r=onur-ozkan

bootstrap: don't resolve symlinks for initial_cargo

I have put the following in my `config.toml`:

```toml
# Includes one of the default files in src/bootstrap/defaults
profile = "compiler"
change-id = 121203

[build]
cargo = "/usr/bin/cargo"
rustc = "/usr/bin/rustc"
rustfmt = "/usr/bin/rustfmt"
```

I have rustup installed from Arch's repos, which has all of the above paths be symlinks to `/usr/bin/rustup`. This works just fine with the `argv[0]` trick that rustup uses.

However, `bootstrap` resolves symlinks to check whether `cargo` exists and then uses the resolved path, so it ends up calling `rustup` directly expecting it to behave like `cargo`. Which it doesn't.

This PR removes the canonicalization step, in turn fixing the issue, but sacrificing a pretty error message. However, this exact thing is checked by `x.py` in advance, so I hope it is not a big deal?

Author: bors