Produce a static VSIX package name on AppVeyor
Produce a static VSIX package name on AppVeyor

This change causes our build script to always produce a VSIX package
called PowerShell-insiders.vsix so that PowerShell scripts can easily
download the packages using the permalink:

https://www.appveyor.com/docs/packaging-artifacts/#permalink-to-the-last-successful-build-artifact

PowerShell/vscode-powershell