[msvc] Remove need for setup scripts on each action.

Talking to Robert S. he mentioned that we could avoid running the vcvars script for each compiler invocation by instead: running the vcvars once on configure, extracting the compiler env vars, converting them to regular build options (i.e. include, libdirs, etc), clearing the env for each compile invocation. This depends on https://github.com/boostorg/build/issues/342