Simple workaround to avoid crash on workspace

[cdussieux]

No description provided.

[luca-della-vedova]

I can't comment on whether the fix is the correct one or not but we are encountering the same issue in rmf_site and this fixes it.

To reproduce, I cloned rmf_site in the workspace and tried a colcon build.

Without this PR the following error is returned:

--- stderr: rmf_site                                                                                                                                                                                         
error: found a virtual manifest at `/usr/local/google/home/lucadv/rmf_ws/src/rmf_site/Cargo.toml` instead of a package manifest
---
Failed   <<< rmf_site [1.22s, exited with code 101]

However after forcing a reinstall and using the PR's colcon-cargo:

pip3 install --user --force-reinstall git+https://github.com/pollen-robotics/colcon-cargo.git

The package builds and runs correctly.
For this reason I'm tentatively approving this to at least unblock the workspace workflows.