[BUG] Build & Packaging task in AzDO should prevent creating packages with wrong extension

[florelis]

In the Build & Packaging task of the MSIX Packaging AzDO Extension, it is possible to create packages with the wrong extension. For example, you can set the output path to "app.msix" while building a bundle package. We should make the task fail in that case, as it causes weird issues down the line.