Ensure ICU is not packaged when Invariant mode is enabled

[steveisok]

When Invariant mode is enabled and we are publishing for iOS / tvOS, we must ensure that no ICU artifacts are produced.

Example:
dotnet publish <required-args> /p:InvariantGlobalization=true

No ICU binaries should be in the publish folder.

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[akoeplinger]

Using the env var is a bad idea, we should rather use the InvariantGlobalization MSBuild property: https://docs.microsoft.com/en-us/dotnet/core/run-time-config/globalization#invariant-mode

[steveisok]

I updated the example. I meant it to be 'pick any option to enable' ;-).