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/libutil/util.hh

@@ -390,6 +390,11 @@ class Callback {
     }
   }
 
+// The unused-variable assert is disabled in this block because the
+// `prev` variables are only used in debug mode (in the asserts).
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wunused-variable"
+
   void operator()(T&& t) noexcept {
     auto prev = done.test_and_set();
     assert(!prev);
@@ -406,6 +411,8 @@ class Callback {
     promise.set_exception(exc);
     fun(promise.get_future());
   }
+
+#pragma clang diagnostic pop
 };
 
 /* Start a thread that handles various signals. Also block those signals