Move nailgun to lib/kframework/bin

[gtrepta]

for #3675

[gtrepta]

@goodlyrottenapple It looks like my change messed up nix somehow, I think it has to do with this snippet here

k/nix/k.nix

Lines 106 to 108 in 41dfa24

	preFixup = lib.optionalString (!stdenv.isDarwin) ''
	patchelf --set-interpreter "$(cat $NIX_CC/nix-support/dynamic-linker)" "$out/bin-unwrapped/ng"
	'';

The error happens on CI here

[dwightguth]

Instead of changing the path in the setenv script, I would recommend just fully qualifying the path to the binary when it gets invoked.

[goodlyrottenapple]

LGTM