Fix up scaffold.sh

Use bash -e instead of single pipeline
Unregister foobar after testing it

yesod/test/scaffold.sh

@@ -1,3 +1,12 @@
-#!/bin/bash -x
+#!/bin/bash -ex
 
-rm -rf foobar && runghc main.hs init && cd foobar && cabal install && cabal install -fdevel && cd ..
+rm -rf foobar
+runghc main.hs init
+
+(
+  cd foobar
+  cabal install
+  cabal install -fdevel
+)
+
+ghc-pkg unregister foobar