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/libstore/nar-info-disk-cache.hh

@@ -25,6 +25,6 @@ class NarInfoDiskCache {
 
 /* Return a singleton cache object that can be used concurrently by
    multiple threads. */
-ref<NarInfoDiskCache> getNarInfoDiskCache();
+std::shared_ptr<NarInfoDiskCache> getNarInfoDiskCache();
 
 }  // namespace nix