fix(3p/nix): Fix all remaining compiler warnings

This compiles under `-Wall -Werror`.

The largest chunk of this change is `final` qualifiers for the various
Nix CLI command structs, which inherit from a Command class that has
more virtual functions than are implemented by each command.

Change-Id: I0925e6e1a39013f026773db5816e4a77d50f3b4a
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1294
Tested-by: BuildkiteCI
Reviewed-by: isomer <isomer@tvl.fyi>
Reviewed-by: kanepyork <rikingcoding@gmail.com>

third_party/nix/src/libexpr/primops.cc

@@ -1680,8 +1680,6 @@ static void prim_partition(EvalState& state, const Pos& pos, Value** args,
   state.forceFunction(*args[0], pos);
   state.forceList(*args[1], pos);
 
-  auto len = args[1]->listSize();
-
   NixList* right = new (GC) NixList();
   NixList* wrong = new (GC) NixList();