Add the beginnings of an auto-deploy script for whitby, intended to
be (eventually) suitable for running automatically in a systemd timer.
The current iteration of the script doesn't actually do any deploying,
but instead takes as an argument a revision, creates a new git worktree
in /tmp with that revision checked out, runs a nix-diff of whitby's
system derivation in the running system and at that closure, puts an
html-rendered version of that diff in the public directory used by
deploy.tvl.fyi, and finally sends a message to IRC via irccat with a
link to that HTML page.
Refs: b/110
Change-Id: Id40525567f8845590c909568befd8d00c07a481c
Reviewed-on: https://cl.tvl.fyi/c/depot/+/3145
Tested-by: BuildkiteCI
Reviewed-by: tazjin <mail@tazj.in>
Reviewed-by: kn <klemens@posteo.de>
