restrict ci on tags

.drone.yml

@@ -10,17 +10,23 @@ steps:
   commands:
   - git submodule init
   - git submodule update --recursive --remote
+  when:
+    event: tag
 
 - name: pandoc
   image: pandoc/alpine:2.14
   commands:
     - ls
     - sh build-pandoc.sh latex > user-manual.tex
+  when:
+    event: tag
 
 - name: build-latex
   image: ghcr.io/xu-cheng/texlive-full:latest
   commands:
     - latexmk -pdf -file-line-error -halt-on-error -interaction=nonstopmode -xelatex user-manual.tex
+  when:
+    event: tag
 
 - name: release
   image: plugins/gitea-release
@@ -32,5 +38,7 @@ steps:
       - user-manual.pdf
     checksum:
       - sha512
-    title: ${DRONE_REPO_BRANCH}
+    title: ${DRONE_COMMIT_REF}
     prerelease: true
+  when:
+    event: tag