Don't use parallel builds in the integration tests

The self-exec method will not work anyway, and it interferes with
getting build output in the stdout log, since individual packages go to
per-package log files (which we cannot see on travis for example).

cabal-install/tests/IntegrationTests2.hs

@@ -281,7 +281,10 @@ mkProjectConfig (GhcPath ghcPath) =
     mempty {
       projectConfigShared = mempty {
         projectConfigHcPath = maybeToFlag ghcPath
-      }
+      },
+     projectConfigBuildOnly = mempty {
+       projectConfigNumJobs = toFlag (Just 1)
+     }
     }
   where
     maybeToFlag = maybe mempty toFlag