Work around setup test failure by specifying Cabal >= 1.18

For packages with custom setup but without explicit setup deps we
currently specify Cabal >= 1.18 because we have trouble talking to older
Cabal versions, but we don't do anything equivalent for packages with
explicit setup deps.

So in this integration test we had a package specifying simply
  custom-setup
    setup-depends: base, Cabal
and so our solver preference for using installed packages (from the
global db) meant that for older ghc we would pick Cabal-1.16 and then
fail to be able to talk to it.

So we don't solve that problem yet, but work around it in this test by
specifying a constraint. Arguably we should add a constraint or
preference for custom setup script with explicit deps too.

cabal-install/tests/IntegrationTests2/build/setup-custom1/a.cabal

@@ -5,7 +5,7 @@ cabal-version: >= 1.10
 
 -- explicit setup deps:
 custom-setup
-  setup-depends: base, Cabal
+  setup-depends: base, Cabal >= 1.18
 
 library
   exposed-modules: A