NuGet doesn't support preserving the executable bit for files in *.nupkgs
(which are just zip files), so .NET implemented a [temporary
workaround](https://github.com/dotnet/sdk/pull/17299) for workload installs,
where they hardcoded files that should be executable.
This isn't really sustainable, and they'll remove their workaround, so we need
to use their supported method of specifying the file permissions: a
data/UnixFilePermissions.xml file.
Fixes https://github.com/xamarin/xamarin-macios/issues/11860.