create build folder only if not existing

.gitlab-ci.yml

@@ -14,7 +14,7 @@ cache:
 conan:
   stage: conan
   script:
-    - mkdir build
+    - mkdir -p build
     - cd build
     - conan remote add bintray https://api.bintray.com/conan/lheili/LibConfig
     - conan user lukas-heiligenbrunner -r bintray -p $CONAN_KEY