Project discovery does not create a valid sysroot

When using rust-analyzer's project discovery, configured very similarly to in [the docs](https://bazelbuild.github.io/rules_rust/rust_analyzer.html), the sysroot that is output does not exist. The only configuration difference is using `--output_user_root` instead of `--output_base`, but I doubt this makes a difference.

This is the offending JSON:

```
"project":{"sysroot":"/home/cameron/.cache/bazel_ra/03e18a5a406e1c59abeb6ec9d1cd7005/external/rules_rust++rust+rust_analyzer_1.86.0_tools","sysroot_src":"/home/cameron/.cache/bazel_ra/03e18a5a406e1c59abeb6ec9d1cd7005/external/rules_rust++rust+rust_analyzer_1.86.0_tools/lib/rustlib/src/library",
```

Either this sysroot is incorrect, or the project discovery has failed to make bazel fetch this repository.