license-file key in Cargo manifest is not read for path dependencies in a workspace

[lopopolo]

Describe the bug
Path dependencies that specify a license-file get the generic MIT license text.

To Reproduce
Run cargo about with Artichoke as of this PR: artichoke/artichoke#1420.

Expected behavior
Path dependencies in the current workspace use the given license file.

Screenshots
If applicable, add screenshots to help explain your problem.

[lopopolo]

It looks like cargo doesn't like it when both license and license-file are specified.

I think what I'd really like is for path dependencies to check the workspace root for the license file.