VCVars: Fix generation if OS is set to WindowsStore

[mkskelet]

Changelog: Fix: Fixes VCVars vcvarsall.bat generation if OS is set to WindowsStore

• added test for VCVars generator targeting armv8 WindowsStore

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[memsharded]

thanks for your contribution @mkskelet

Looking good, I have launched the CI to test the change.

Could you please have a look and sign the CLA, so we can merge this? Thanks!