This PR resolves a crash when running the linker on publishing iOS extensions.
The crash would occur here in failing to resolve corelib.
The reason this would fail was System.Private.CoreLib.dll was not in input_assemblies.
This was because we were passes the set of reference assemblies not the expected 'real' ones, and those do not include CoreLib.
After a bunch of digging, this was because _ComputeManagedRuntimePackAssembliesIfSelfContained target was not being set as a condition of _ComputeAssembliesToPostprocessOnPublish.
_ComputeManagedRuntimePackAssembliesIfSelfContained happened to be the place these were added, and wasn't being set since it has a condition of $(SelfContained) == 'true'
Now confusingly SelfContained WAS being set to true, but only in the targets file, which was too late, as it was checked in a 'global' property group outside of a target.
This means we'd fail to set SelfContained until after the condition, and not run.
This was verified by setting /p:SelfContained=true to true.
I also looked at removing the condition above, since https://github.com/dotnet/runtime/issues/54406 is fixed, however this caused project that didn't set RuntimeIdentifier to fail.
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