Allow env var in apiKeys config

[lonix1]

NuGet Product(s) Involved

dotnet.exe, NuGet SDK

The Elevator Pitch

There are places where env vars are supported, but not for those bits that are useful for ci/cd.

Please allow us to read from an env var in this part of the nuget.config:

<apikeys>
  <add key="https://gitea.example.com/api/packages/myorg/nuget/index.json" value="%ACCESS_TOKEN%" />
</apikeys>

We could define the ACCESS_TOKEN env var in the pipeline, and nuget would detect it and use it for that specific nuget source.

(The nuget.config section with username/password is not useful in a ci/cd environment - we need to use api keys.)

Additional Context and Details

Using nuget in a ci/cd environment is a major pain, and has been for years (especially on linux, which is were most devops takes place). Please don't forget us.