Write Gitpod `app.ini` only once (#23192)

Before this change, the `app.ini` would get overwritten on each
workspace start, confusing Gitea. It asked for reinstallation each time.
This makes sure the file is written only once by checking it does not
exist before creating it.

---
[Review without whitespace
diff](https://github.com/go-gitea/gitea/pull/23192/files?w=1)

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Co-authored-by: delvh <dev.lh@web.de>

.gitpod.yml

@@ -10,9 +10,12 @@ tasks:
   - name: Run backend
     command: |
       gp sync-await setup
+      if [ ! -f custom/conf/app.ini ]
+      then
         mkdir -p custom/conf/
         echo -e "[server]\nROOT_URL=$(gp url 3000)/" > custom/conf/app.ini
         echo -e "\n[database]\nDB_TYPE = sqlite3\nPATH = $GITPOD_REPO_ROOT/data/gitea.db" >> custom/conf/app.ini
+      fi
       export TAGS="sqlite sqlite_unlock_notify"
       make watch-backend
   - name: Run frontend