Using the lockfile hash as cache key did not make much sense, since
changes to Unstraightened itself change what we fetch.
And a static restore key means we never flush our cache: that might be
ok if cache size was reasonable, but it is not (over 3 GiB).
Switch to using the date as cache key, and only restore caches from the
current month.
Add tarballs as a second cache.
I considered caching all of ~/.cache/nix, but do not want to find out
the hard way any (new) caches are not portable if I add CI for other
platforms. The tarball-cache seems to be another git tree, so it should
be ok.
CI is failing with 'Error: Invalid("no nixpkgs dependency found for
specified key: ")`, probably because of this leftover start at multiple
nixpkgs inputs.
Remove it.
This is only a starting point. For this to be more useful, we need to
build for multiple Emacsen. But I want to get a baseline for the size of
the closure first (as well as confirming it works in the first place).
