qt-mobility: move to homebrew-headonly tap.

https://github.com/Homebrew/homebrew-headonly

Relies on unreleased Git versions to build for Lion or above.

Closes #22285.
cabal-install: update to 1.18.0.1, sandbox build.

Use sandboxing support rather than ~/.ghc for build.

Closes #22416.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>