Add an ENV Variable for Package Signing Verification on .NET 5+ Linux/MAC

[erdembayar]

Details about Problem

NuGet product used (NuGet.exe | dotnet.exe):

Product version:

Worked before? If so, with which NuGet version: N/A

Repro steps and/or sample project

Add an environment variable to opt-in to the package signing verification on .NET 5.
Env var name is DOTNET_NUGET_SIGNATURE_VERIFICATION and to enable it set value to TRUE. Please note both env var name and value are case sensitive, must be all capital letters.
This is to provide an opt-in / troubleshooting experience for Mac/Linux based on the reversion of:
NuGet/NuGet.Client#3979
We intend this ENV for testing and don't guarantee compatibility.

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