it looks like mautrix-signal reads the appserver token (AS_TOKEN) from
its config file -- which we place in the nix store. as such, we have no
easy way of getting the token from registration.yaml over to
mautrix-signal. this is presumably what the environmentFile stuff is
meant for, but it doesn't *really* help much.
i think it makes sense to pursue coffeetables' nix-matrix-appservices
module, which has good-looking AS_TOKEN support:
<https://gitlab.com/coffeetables/nix-matrix-appservices>
