Add option to not populate ENV in authenticate

[mortenpi]

authenticate(server) will set ENV["JULIA_PKG_SERVER"]. This side-effect can be surprising when authenticate is used as a library function in other code.

It's probably a reasonable default, especially for interactive workflows. But could we add an option to opt out of this behavior in library code? Something like authenticate(server; update_env=true), and the calling code could call it with update_env=false?

[pfitzseb]

We could also try to be cute here and rename the current function to authenticate!.

[mortenpi]

That would be a breaking change though I guess?

[pfitzseb]

True.