testing/nix: don't install a custom profile file
Instead use the one provided by upstream. This will make it easier
to upgrade Nix in the future and also aligns our Nix setup with
what is documented in upstream's Nix Manual (see also: !48275 (merged)). Also note that for
multi-user setups utilizing nix-daemon we need to use the
etc/profile.d/nix-daemon.sh file, not etc/profile.d/nix.sh.
With the new setup we still need to set NIX_REMOTE=daemon which
is now achieved through a separated file [1].