AppVeyor: add Wix to PATH

.appveyor.yml

@@ -15,6 +15,14 @@ install:
 - ps: Install-Product node $env:nodejs_version $env:platform
 - set PATH=%APPDATA%\npm;%PATH%
 - yarn
+- ps: |
+    $wixToolsetBinPath = ";C:\Program Files (x86)\WiX Toolset v3.11\bin;"
+    $env:PATH = $env:PATH + $wixToolsetBinPath
+    [Environment]::SetEnvironmentVariable("Path", $env:PATH, [EnvironmentVariableTarget]::Machine)
+    if ((Get-Command "light.exe" -ErrorAction SilentlyContinue) -eq $null)
+    {
+      Write-Host "Unable to find light.exe in your PATH"
+    }
 
 test_script:
 - node --version