gen_rust_project not downloading/generating a .so file


There is a crate-spec file present in the dir:
```
ls ~/.cache/bazel/_bazel_robertc/8b0aece0fa823e87b78e3a123965ccb1/execroot/_main/bazel-out/linux_amd64-opt-exec-ST-b021421c07d4/bin/external/_main~crate_repositories~crates_vendor__extend-1.2.0/
extend.rust_analyzer_crate_spec.json
```

but nothing else...

The spec file refers to the .so file:
```
"proc_macro_dylib_path": "__EXEC_ROOT__/bazel-out/linux_amd64-opt-exec-ST-b021421c07d4/bin/external/_main~crate_repositories~crates_vendor__extend-1.2.0/libextend-2251390687.so",
```

Doing a bazel clean + gen_rust_project with different --remote_download_outputs= options doesn't affect the presence of the .so.