[Feature Request] Export the VSINSTALLDIR env variable too.

[anr2me]

Apparently the VSINSTALLDIR/VCINSTALLDIR env variables doesn't exist on windows runner (ie. windows-2019/windows-latest)

Example use case, to easily locate clang toolset during: msbuild MyProj.vcxproj /p:CLToolExe=clang-cl.exe /p:CLToolPath=c:\whatever\path\to\the\tool

Where by default, clang-cl.exe is located in *%VCINSTALLDIR%\Tools\Llvm\bin\* and *%VCINSTALLDIR%\Tools\Llvm\x64\bin\*.

PS: We can make do with just VSINSTALLDIR exported, since the VCINSTALLDIR will most likely be at %VSINSTALLDIR%\VC