(ci) prevent optionally mounted calendar directory to be deleted during build

.gitlab-ci.yml

@@ -101,6 +101,9 @@ build:
     rules:
         - if: $CI_COMMIT_REF_PROTECTED == 'true'
     image: node:22.14.0
+    variables:
+        # calendar can be mounted to supply its containing build-time sources
+        GIT_CLEAN_FLAGS: '-fx -e calendar'
     script:
         # see https://docs.gitlab.com/ee/ci/jobs/ssh_keys.html#ssh-keys-when-using-the-docker-executor
         - 'command -v ssh-agent >/dev/null || ( apt-get update -y && apt-get install openssh-client -y )'