Prevent local OCaml from interfering on AppVeyor

The wrong DLLs can end up being loaded while building OCaml :-/

Author: dra27

Diff for: appveyor_build.sh

@@ -119,4 +119,6 @@ git remote add local $(echo "$APPVEYOR_BUILD_FOLDER"| cygpath -f -) -f --tags
 run "git checkout $APPVEYOR_REPO_COMMIT" git checkout merge
 cd ..
 
+mv $OCAMLROOT $OCAMLROOT-Disabled
 run "make world" $MAKEOCAML flexdll world
+mv $OCAMLROOT-Disabled $OCAMLROOT