Does not work properly with git dependencies when the dep is a workspace

[xlambein]

Describe the bug
When Cargo.toml specifies a git dependency which points to a Cargo workspace, the build fails because of a bad cargo/config file.

To Reproduce
Make a Cargo project that depends on a crate located in a workspace hosted on git.

Expected behavior
The build should work.

Linked issue
This issue has already been reported on upstream crate2nix: nix-community#207.