feat(tools/depot-nixpkgs-update): tool to update our pin

Very simplistic tool to aid with updating our nixpkgs channel pins:
This tool prints the correct two `*Hashes` sets to stdout, so you can
easily delete the appropriate lines in `third_party/nixpkgs/default.nix`,
run

    ./bin/depot-nixpkgs-update | wl-copy

and paste it into your editor.

Doing this fully automatically would be possible, but would either
a) require changing `default.nix`, so it is regex-able more easily,
b) touching something like rnix-parser which I have no motivation to
at the moment or
c) searching for the old hashes and replacing them with the new
ones. This may be a simple and worthwhile improvement in the future.

Change-Id: I4df44e3827ce9ff6e4fe2d336c08016d799e21a7
Reviewed-on: https://cl.tvl.fyi/c/depot/+/3252
Tested-by: BuildkiteCI
Reviewed-by: tazjin <mail@tazj.in>

bin/__dispatch.sh

@@ -19,6 +19,9 @@ case "${TARGET_TOOL}" in
   depot-build)
     attr="tools.depot-build"
     ;;
+  depot-nixpkgs-update)
+    attr="tools.depot-nixpkgs-update"
+    ;;
   gerrit)
     attr="tools.gerrit-cli"
     ;;