chore: remove unnecessary result on join_unix_path (#7506)
### Description
Realized that this shouldn't ever fail unless we bypass the checks done
in the constructors for both `AbsoluteSystemPath` and
`RelativeUnixPath`. The only failure point is converting the `PathBuf`
to `AbsoluteSystemPathBuf` which itself has [two failure
points](https://github.com/vercel/turbo/blob/main/crates/turborepo-paths/src/absolute_system_path_buf.rs#L208).
- Conversion from stdlib `PathBuf` to `Utf8PathBuf` which only fails if
the [path can't be represented in
utf8](https://docs.rs/camino/latest/src/camino/lib.rs.html#156)
- `AbsoluteSystemPathBuf` constructor which only fails if the input path
isn't absolute
### Testing Instructions
Existing test suite, my reasoning above
Closes TURBO-2428
