Add vcvars64.bat env variables

The location of ‘vcvars64.bat’ can vary from install to install, and depends on what VS package is used.  At present, Windows images only have one version installed, but that could change.

Having env variables like ‘vcvars64’ (latest), ‘vcvars64_19’ or ‘vcvars64_16’ might be helpful.  The variables would be the full string/path needed to run the command.

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