Fix Appveyor CI.

appveyor.yml

@@ -36,6 +36,9 @@ install:
     - alex  --version
     - happy --version
     - stack exec -- ghc-pkg list
+    - mkdir _build
+    - cd _build
+    - mkdir hadrian
 
 build_script:
     - cd C:\msys64\home\ghc\hadrian