Work with NIX_CONF_DIR

[allenap]

Nix itself doesn't have a story for saying "I want some project specific options", but firstaide could do something useful with NIX_CONF_DIR to make it work.

Quick idea: firstaide could write out a nix.conf in some situations:

Global NIX_CONF_DIR	Project-specific NIX_CONF_DIR	What do to
unset	unset	Nothing
unset	project/etc	Write new nix.conf: include /etc/nix, include project/etc
/some/thing	unset	Nothing
/some/thing	project/etc	Write new nix.conf: include /some/thing, include project/etc

Why include /etc/nix? From nix.conf(5): 'this is the "system-wide configuration file sysconfdir/nix/nix.conf'. It doesn't seem possible or at least obvious how to find out what Nix's notion of sysconfdir is, so we assume it's /etc for now.