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

Author: daviwil

vscode-powershell.build.ps1

@@ -93,13 +93,16 @@ task Package {
         Copy-Item -Recurse -Force ..\PowerShellEditorServices\module\PowerShellEditorServices .\modules
     }
 
-    Write-Host "`n### Packaging PowerShell-$($script:ExtensionVersion).vsix`n" -ForegroundColor Green
+    Write-Host "`n### Packaging PowerShell-insiders.vsix`n" -ForegroundColor Green
     exec { & node ./node_modules/vsce/out/vsce package }
+
+    # Change the package to have a static name for automation purposes
+    Move-Item .\PowerShell-$($script:ExtensionVersion).vsix .\PowerShell-insiders.vsix
 }
 
 task UploadArtifacts -If { $env:AppVeyor } {
 
-    Push-AppveyorArtifact .\PowerShell-$($script:ExtensionVersion).vsix
+    Push-AppveyorArtifact .\PowerShell-insiders.vsix
 }
 
 # The default task is to run the entire CI build