Revert "nixUnstable: rename to nix-unstable"

This commit effectively makes it impossible to upgrade to nixUnstable
with nix-env without mucking about with priorities, as you can't
uninstall the old nix transactionally with the new nix and if you
uninstall the old one first you no longer have nix at your disposal to
install the new one.

This reverts commit 9711aac6.