Stop making source modifications during build

[MichaelSimons]

During the build several source files are modified. E.g. NuGet feeds are injected, the SDK in the global.json is set to a common version, etc. These didn't cause a problem within a tarball environment but now with the VMR, the present a usability issue for developers making changes intended to be submitted as these infra changes should not be committed.

This PR is a tracking issue to address the following issues:

• Stop making src modifications to every repo's NuGet.config #3170
• Stop making src modifications to every repo's global.json #3169
• Stop injecting NoWarns into repo's Directory.Build.props #3117
• Stop making src modifications to every repo's eng/common/tools.sh #3171
• Avoid having to edit script files in source when building #4038

[tkapin]

This is currently paused, might require additional product changers to not modify the global.json.