Do local tincification in $out

[Chris Warburton]

In tincify we either use a "global" cache, which lives in /tmp and is
shared by all runs but may cause permission problems on Hydra, or we use
a "local" cache which lives in the package's build dir.

It looks like we need to load the source of some packages (notably:
any "extras" given to tincify), but if these were cached "locally" then
the build dir they lived in is long gone, and hence so is their source.

Instead, we should put the cache in some "$out" directory; maybe of a
separate derivation, so that it persists.