Support VS 2019 from Native Tools Command Prompt.

[john-preston]

Right now GYP isn't detecting VS 2019 if only it is installed in the system (without VS 2017 or earlier).

This PR allows GYP to work from one of the provided batch environments (like "x86 Native Tools Command Prompt for VS2019").

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[john-preston]

@googlebot I signed it!

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