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/tests/language-tests.cc

@@ -174,7 +174,7 @@ TEST_P(EvalFailureTest, Fails) {
   EvalState state({}, ref<Store>(store));
   auto path = GetParam();
 
-  Expr* expr;
+  Expr* expr = nullptr;
   EXPECT_NO_THROW(expr = state.parseExprFromFile(GetParam().string()))
       << path.stem().string() << ": should parse successfully";