Add "useLatest" arg to defs.nix

[Chris Warburton]

Pass this flag into anything which uses paths, like <some-project>. If
the flag is true, always use the path (don't even check if we have it!);
if it's false, always use the static git revision.

In release.nix, make two versions: stable and latest, with this flag set
to false and true, respectively.

That way, Hydra will build both versions.