Accept command line parameters from environment variable

Some tools have a handy (hidden?) feature that you can pass them command line parameters through the environment.

For example, the Microsoft C++ compiler accepts them through `_CL_` and the linker through `_LINK_`. This can be super handy for adding temporary flags when you don't (easily) control the invocation such as when it's inside a script, batch file, or makefile. For example in the case of the compiler you can use it to preprocess a file with the same defines as the real build, or to get timing information.

This would also be useful for MSBuild. Right now I'm looking at the build of dotnet/runtime. I want to temporarily add a verbose logger like `/flp6:v=diag`. Right now to do this I have to dig through a chain of batch files, shell scripts, powershell scripts, and/or cmake files to find the 10 or more places where MSBuild may be launched within these and paste in the flag. This also adds noise in my diff. Later, I have to revert it as well.

If I could just do `set _MSBUILD_=/flp6:v=diag` it would solve all these problems.

I offer a proposed implementation in case you believe this is worthwhile.