Step two for #496.
The idea is to allow `nix-build --arg config.allowUnfree true` do the
right thing in the future. However, that's a breaking change since
people might be relying on the ability to set `"config.allowUnfree"` as
attribute-name when auto-calling a Nix-expression.
As a first step, a warning got introduced in 2.92, the next step is now
to reject this usage in 2.94 and await feedback if any so that we can do
the change in a future Lix release.
Change-Id: I6e38fafe26e234204f5bba2a3a4c1da10f80e5f2
Maximilian Bosch
3a6414760e