rattler.solve sometimes can't select pinned local versions

[calebzulawski]

Checklist

• I added a descriptive title
• I searched open reports and couldn't find a duplicate

What happened?

I have a spec mypackage ~=2.0. If I have a previously installed version 2.0.0+fastbuild passed via pinned_packages, I sometimes get one of two errors:

exceptions.SolverException: Cannot solve the request because of: No candidates were found for mypackage ~=2.0.

or

exceptions.SolverException: Cannot solve the request because of: The following packages are incompatible
└─ mypackage ~=2.0 can be installed with any of the following options:
   └─ mypackage 2.0.0 | 2.0.0
├─ mypackage 2.0.0+fastbuild is locked, but another version is required as reported above
├─ mypackage 2.0.0+fastbuild is locked, but another version is required as reported above

Additional Context

In some of these cases, the pinned package is not in channels passed to rattler.solve (because the package was originally installed out of another channel). I think pinned packages need to be considered regardless of if they're present in any of the channels.