Appveyor variable fix attempt #2'

appveyor.yml

@@ -1,5 +1,5 @@
 clone_folder: c:\projects\jsoncpp
-build_folder: '{clone_folder}\build\release'
+
 environment:
   matrix:
     - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2015
@@ -13,8 +13,8 @@ environment:
 
 build_script:
   - cmake --version
-  - mkdir -p '{build_folder}'
-  - cd '{build_folder}'
+  - mkdir -p '{clone_folder}\build\release'
+  - cd '{clone_folder}\build\release'
   - cmake -G "%CMAKE_GENERATOR%" -DCMAKE_INSTALL_PREFIX:PATH=%CD:\=/%/install -DBUILD_SHARED_LIBS:BOOL=ON '{clone_folder}'
   # Use ctest to make a dashboard build:
   # - ctest -D Experimental(Start|Update|Configure|Build|Test|Coverage|MemCheck|Submit)