Update pack to allow content to be included when targeting multiple frameworks

[jgonz120]

Bug

Fixes: https://github.com/NuGet/Home/issues/11467 Fixes: https://github.com/NuGet/Home/issues/6792

Description

Updated targets to run _GetPackageFiles on inner build. This will allow for content files to be included in the package when packing.

PR Checklist

• [x] Meaningful title, helpful description and a linked NuGet/Home issue
• [x] Added tests
• [ ] Link to an issue or pull request to update docs if this PR changes settings, environment variables, new feature, etc.

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