For a multi-targeted build, the --output command-line option will cause the builds for multiple target frameworks to go to the same output path. This will likely cause a build failure due to files being in use, or an incorrect build output. See #13359.
We should do something to avoid this pit of failure. Some possibilities:
- Generate a build error saying this isn't supported
- Automatically append the TargetFramework to the specified output path
- ???