internal: tool discovery prefers sysroot tools
Fixes https://github.com/rust-lang/rust-analyzer/issues/15927, Fixes https://github.com/rust-lang/rust-analyzer/issues/16523
After this PR we will look for `cargo` and `rustc` in the sysroot if it was succesfully loaded instead of using the current lookup scheme. This should be more correct than the current approach as that relies on the working directory of the server binary or loade workspace, meaning it can behave a bit odd wrt overrides.
Additionally, rust-project.json projects now get the target data layout set so there should be better const eval support now.
5e1b09bb76