ci: pin wix extension version

There is a wix bug that causes the latest version of an extension to be
installed with `wix extension add`, even if the latest version is a
pre-release (such as 5.0.0-rc.2).

This attempts to pin the exension version to match the installed wix
version.

.github/workflows/ci.yml

@@ -250,7 +250,7 @@ jobs:
           dotnet tool install --global wix --version 4.0.5
       - name: Install Wix UI Extension
         run: |
-          wix extension add -g WixToolset.UI.wixext
+          wix extension add -g WixToolset.UI.wixext/4.0.5
       - name: Build MSI Installer
         shell: msys2 {0}
         run: |