Auto merge of #56194 - eddyb:top-unhack, r=alexcrichton

Try to make top-level Cargo.toml work without __CARGO_TEST_ROOT.

The last failures I noticed before adding the `__CARGO_TEST_ROOT` hack were in `src/test/run-make/thumb-none-{cortex-m,qemu}`, and had to do with the fact that the top-level `Cargo.toml` uses nightly Cargo features.

If that's the only problem, this PR should work, and we can figure out how to unbreak RLS.