jailbreak-cabal: build ghc-7.10.x version from source again

https://github.com/peti/jailbreak-cabal/issues/9 forced us to use a
binary version of jailbreak-cabal built with ghc 7.8.x in the 7.10.x
package set, which is awkward. Upstream fixed the underlying issue
in https://github.com/haskell/cabal/issues/2598, but unfortunately
that fix didn't make it into the Cabal 1.22.4.0 release shipped by
ghc 7.10.2.

Now, we work around that issue by building jailbreak-cabal with an
unreleased development snapshot of Cabal in the ghc 7.10.x package
set.

Closes https://github.com/NixOS/nixpkgs/issues/8901.