Do not include solution folders in get solution projects

Currently, the RegEx used in GetSolutionProjects to identify projects in a solution also matches solution folders.

Repro steps:
- From any open solution, open the Solution Explorer window
- Right click on the solution and select `Add > New Solution Folder`
- Save the solution
- Run MSBuild using the GetSolutionProjects target.

The folder added to the solution will be listed as one of the projects of the solution.