don't mkdir build if it exists

.gitlab-ci.yml

@@ -48,7 +48,7 @@ windows:
     - 'dir Env:'
     # make the CMake build directory
     - echo "----------------------------------------"
-    - mkdir -Force "$BUILD_DIR"
+    - if(!(Test-Path -path $BUILD_DIR)) { mkdir -Force $BUILD_DIR }
     - cd "$BUILD_DIR"
     - pwd
     - echo "#--- CONFIGURE ---"