Sign UWP artifacts (dotnet/core-setup#2770)

Sign UWP artifacts (dotnet/core-setup#2770)

* The SignUwpFiles target wasn't attached anywhere. Move the files it wants to sign into the SignBinaries target.

* Updated baseline.props

* appx file is not produced during the build binaries step and is not
getting baselined

* Add signing of appx to "Sign Msi and cab" step

* Remove appx file signing

* Ignore casing for baseline comparison

* Fix casing

* Fix parens

Commit migrated from https://github.com/dotnet/core-setup/commit/73e57cebd9c71728d409407b6c4b7f7f9a56fa02