NMake Makefiles: Allow specifying a previous stable Rust toolchain

Hi,

This attempts to try to give a an option to be used in NMake builds for a workaround for issue #968 (closed) when the latest stable Rust release (which is installed via rustup update) had unexpected changes or issues that break the build by allowing one to specify an installed previous-stable release version of the Rust toolchain that is to be used for the build.

With blessings, thank you!