r/gitea: Set WORK_DIR in config

Gitea complains if the `WORK_DIR` setting is not set.  It tries to set
it itself, but fails because the configuration is read-only.  The value
it uses is incorrect anyway (`/usr/local/bin`, since that's where the
`gitea` executable is).

roles/gitea/templates/app.ini.j2

@@ -1,6 +1,7 @@
 APP_NAME = Gitea: Git with a cup of tea
 RUN_USER = gitea
 RUN_MODE = prod
+WORK_PATH = /var/lib/gitea
 
 [security]
 INTERNAL_TOKEN = {{ gitea_internal_token }}