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/parser.y

@@ -11,7 +11,10 @@
 %expect 1
 %expect-rr 1
 
-%code requires { #include "libexpr/parser.hh" }
+%code requires {
+#define YY_NO_INPUT 1 // disable unused yyinput features
+#include "libexpr/parser.hh"
+}
 
 %{