Skipping unsupported projects does not exclude NuGet imports during restore

[jeffkl]

This new feature breaks the build because GetProjectsToSkip runs before Restore and does not ignore missing imports from packages, causing errors like error MSB4019: The imported project "C:\msbuild\CompileModule.proj" was not found. Confirm that the expression in the Import declaration "\msbuild\CompileModule.proj" is correct, and that the file exists on disk. 4.0.0 breaks the build, I'm rolling back to 3.1.6

Originally reported at https://github.com/microsoft/MSBuildSdks/issues/438#issuecomment-1546260640