nix-index: Wrap with nix-env in PATH (#51165)

This fixes an impurity in nix-index: Previously it would take the nix-env
binary from the users PATH. I discovered this while trying to run nix-index in a
systemd service, which by default doesn't have nix-env in its path. The
errors it threw were not informative at all and it took me hours to
finally figure out the reason.