Fix literal \n in the release changelog body

The bash double-quoted BODY="...\n..." assignment never interpreted
\n as a newline, so Gitea rendered the literal backslash-n text
instead of a line break (visible on the 2.1.3 release). Build it
with printf instead, which does expand \n.

.gitea/workflows/ci.yml

@@ -124,9 +124,9 @@ jobs:
           fi
 
           COMMIT_LINK="https://git.djeex.fr/Djeex/lumeex/commit/${{ github.sha }}"
-          BODY="**$CATEGORY**\n\nCommit: $COMMIT_LINK"
+          BODY=$(printf '**%s**\n\nCommit: %s' "$CATEGORY" "$COMMIT_LINK")
           if [ -n "$PR_LINK" ]; then
-            BODY="$BODY\nPR: $PR_LINK"
+            BODY=$(printf '%s\nPR: %s' "$BODY" "$PR_LINK")
           fi
 
           jq -n \