qgroundcontrol: Fix running of {pre,post}Configure

Commit 0055c6aa

 introduced a new preConfigure hook that sets the right
qmake path. Unfortunately the mkDerivation attributes of qgroundcontrol
override the whole configurePhase, so this hook isn't run at all.

Instead of using pushd/popd, I'm making it a bit more readable by just
spawning a subshell with the right working directory.

This fixes the build of qgroundcontrol and it now successfully compiles
on my machine.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>