Correct invalid path to project file
Correct invalid path to project file

It previously extended the relative path from the .sln file to the .proj file using the current directory, whereas it should have used the directory with the .sln.

Fixes #5712

mono/msbuild,cdmihai/msbuild,AndyGerlicher/msbuild,mono/msbuild,rainersigwald/msbuild,AndyGerlicher/msbuild,rainersigwald/msbuild,cdmihai/msbuild,AndyGerlicher/msbuild,mono/msbuild,sean-gilliam/msbuild,sean-gilliam/msbuild,sean-gilliam/msbuild,rainersigwald/msbuild,cdmihai/msbuild,rainersigwald/msbuild,sean-gilliam/msbuild,cdmihai/msbuild,mono/msbuild,AndyGerlicher/msbuild