doctest doesn't like being cross compiled

[quasicomputational]

The path to the GHC that doctest invokes is determined at build time to be the GHC that is building it. This is obviously unfriendly to cross compilation.

I remember being convinced that this was for good reasons... but not what those reasons were, so it's possible I'm mistaken here. If there are good reasons, they should be written down for later reference and, if there aren't, ideally doctest would be changed to be more cross friendly.

This is possibly related to #282 and haskell/actions#30.