Auto merge of #11023 - dpc:cargo-cmd-path-order, r=epage

Auto merge of #11023 - dpc:cargo-cmd-path-order, r=epage

Do not add home bin path to PATH if it's already there

This is to allow users to control the order via PATH if they so desire.

Tested by preparing two different `cargo-foo` scripts in `$HOME/.cargo/bin` and `$HOME/bin`:

```
> env PATH="/usr/bin/:$HOME/bin:$HOME/.cargo/bin" ./target/debug/cargo foo
Inside ~/bin/
> env PATH="$HOME/.cargo/bin:/usr/bin/:$HOME/bin" ./target/debug/cargo foo
Inside ~/.cargo/bin/
> env PATH="/usr/bin/:$HOME/bin" ./target/debug/cargo foo
Inside ~/.cargo/bin/
```

and trailing slash:

```
> env PATH="$HOME/.cargo/bin/:/usr/bin/:$HOME/bin" ./target/debug/cargo foo
Inside ~/.cargo/bin/
> env PATH="/usr/bin/:$HOME/bin:$HOME/.cargo/bin/" ./target/debug/cargo foo
Inside ~/bin/
```

Fix https://github.com/rust-lang/cargo/issues/11020