Use `add-source` instead of linking cabal.sandbox.config

As suggested by mpickering

build.cabal.sh

@@ -61,11 +61,7 @@ else
     # Initialize sandbox if necessary
     if ! ( "$CABAL" sandbox hc-pkg list > /dev/null 2>&1); then
         "$CABAL" sandbox init
-        ( cd ../libraries/Cabal/Cabal
-          ln -s "$absoluteRoot/cabal.sandbox.config" cabal.sandbox.config
-          "$CABAL" install
-          rm cabal.sandbox.config
-        )
+        "$CABAL" sandbox add-source ../libraries/Cabal/Cabal
         "$CABAL" install                \
             --dependencies-only         \
             --disable-library-profiling \