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 source-build#3170
• Stop making src modifications to every repo's global.json #1004
• Stop injecting NoWarns into repo's Directory.Build.props source-build#3117
• Stop making src modifications to every repo's eng/common/tools.sh source-build#3171
• Avoid having to edit script files in source when building #1020