Hadrian: Allow passing CABFLAGS into build.cabal.sh

Setting `CABFLAGS=args` will pass the additional arguments to cabal
when it is invoked.

hadrian/build.cabal.sh

 #!/usr/bin/env bash
 
 CABAL=cabal
-CABFLAGS="--disable-documentation --disable-profiling --disable-library-profiling"
+CABFLAGS="--disable-documentation --disable-profiling --disable-library-profiling $CABFLAGS"
 
 # It is currently more robust to pass Cabal an absolute path to the project file.
 PROJ="$PWD/hadrian/cabal.project"