ci: write the mirror deploy key with a trailing newline (#1748)

OpenSSH rejects key files without a trailing newline ('error in libcrypto'); the first post-merge run of the mirror sync failed on exactly this. printf %s does not add one.

Co-authored-by: kingpanther13 <kingpanther13@users.noreply.github.com>
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

Author: kingpanther13

.github/workflows/sync-integration-mirror.yml

@@ -29,7 +29,7 @@ jobs:
           DEPLOY_KEY: ${{ secrets.INTEGRATION_MIRROR_DEPLOY_KEY }}
         run: |
           mkdir -p ~/.ssh
-          printf -- "%s" "$DEPLOY_KEY" > ~/.ssh/mirror_key
+          printf -- "%s\n" "$DEPLOY_KEY" > ~/.ssh/mirror_key
           chmod 600 ~/.ssh/mirror_key
           ssh-keyscan github.com >> ~/.ssh/known_hosts 2>/dev/null