nixos/tests/chromium: Re-add map for all channels

This partially reverts f2d24b98

.

Instead of disabling the channels via removing the channel mapping from
the tests themselves, let's just explicitly reference the stable test in
release.nix. That way it's still possible to run the beta and dev tests
via something like "nix-build nixos/tests/chromium.nix -A beta" and
achieve the same effect of not building beta and dev versions on Hydra.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>