Fix AppVeyor checkout directory

Author: stuartmorgan-g

.appveyor.yml

@@ -20,7 +20,7 @@ install:
   # None of the packaged channels are new enough for the current state
   # of FDE, so for now clone master instead.
   #- ps: build\ci\install_flutter.ps1 $env:APPVEYOR_BUILD_FOLDER\..
-  - git clone -b master https://github.com/flutter/flutter.git
+  - git clone -b master https://github.com/flutter/flutter.git %APPVEYOR_BUILD_FOLDER%\..
 
 build_script:
   - msbuild "example\windows_fde\Example Embedder.sln"