Building metadata projects outside vscmd results in failure to find cl.exe

Just creating an official issue to track fixing a long time annoyance. `dotnet build` in a vanilla Terminal session results in midl.exe/cl.exe failures due to tooling paths not getting wired up correctly.

Reproduce dotnet build in a vanilla Terminal session and compare it with a vscmd session, focusing on how tooling paths are wired. Done means the build can locate both cl.exe and midl.exe outside vscmd.

Written by the indexing model from the issue text.