Add 'stable' flag

[Chris Warburton]

Lots of our definitions use things like "latestGit" and "tincify", which
aren't necessarily deterministic. On the other hand, we've built up so
many useful helpers that many of our repos now depend on a 'fixed
version' of nix-config. Those versions, of course, aren't particularly
'fixed' if they call out to latestGit.

We should add an optional flag, defaulting to true, for whether "stable"
(i.e. fixed) versions of everything should be used. We can check both
possibilities with release.nix.

As a start, we should make the 'stable' version of latestGit throw an
assertion error, since latestGit is itself unstable. We should make a
single exception: provide the latest version of nix-config in a package.
That way, we can fetch a stable nix-config with fetchgit, and pick out
of it the latest, unstable nix-config; without having to write the same
boilerplate over and over to fetch the latest version.

That 'latest nix-config' package should only provide the 'unstable'
version (i.e. with 'stable' flag set to 'false'), to deter anyone who
wants stability from using such an output of latestGit.