External relative path dependencies not working

[criemen]

With #3025, we ought to be able to use relative path dependencies.
That's not working for me.
You can check my code out in https://github.com/github/codeql/tree/criemen/rules-rust-upg, and run
bazel run //misc/bazel/3rdparty:vendor_ruby_deps.
I'm getting

INFO: Running command line: bazel-bin/misc/bazel/3rdparty/vendor_ruby_deps.sh

error: failed to load manifest for workspace member `/tmp/.tmpFAhxxI/extractor`
referenced by workspace at `/tmp/.tmpFAhxxI/Cargo.toml`

Caused by:
  failed to load manifest for dependency `codeql-extractor`

Caused by:
  failed to read `/tmp/shared/tree-sitter-extractor/Cargo.toml`

Caused by:
  No such file or directory (os error 2)

Error: Failed to fetch crates for lockfile: exit status: 101