Use new-build for solver-debug-flags Travis job.

travis-solver-debug-flags.sh

 #!/bin/sh
 
 # Build cabal with solver debug flags enabled.
-#
-# We use a sandbox, because cabal-install-1.24.0.0's new-build command tries to
-# build tracetree's dependencies with the inplace Cabal, which leads to compile
-# errors. We also need to skip the tests, because debug-tracetree prints the
-# whole solver tree as JSON.
+# We need to skip the tests, because debug-tracetree prints the whole solver
+# tree as JSON.
 
 cabal update
-cd cabal-install
-cabal sandbox init
-cabal sandbox add-source ../Cabal
-cabal install --dependencies-only --constraint "cabal-install +debug-tracetree +debug-conflict-sets"
-cabal configure --ghc-option=-Werror --constraint "cabal-install +debug-tracetree +debug-conflict-sets"
-cabal build
+cabal install happy
+cabal new-build exe:cabal --constraint "cabal-install +debug-tracetree +debug-conflict-sets"