Fix YAML parse error in the gem-publish step

The credentials printf line contains ': ' which YAML reads as a mapping;
use a literal block scalar for that command.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

.woodpecker.yaml

@@ -43,7 +43,8 @@ steps:
       - cd libs/ruby/warp_engine
       - gem build warp_engine.gemspec
       - mkdir -p ~/.gem
-      - printf -- '---\n:https://git.teletypegames.org/api/packages/tools/rubygems: Bearer %s\n' "$${FORGE_TOKEN}" > ~/.gem/credentials
+      - |
+        printf -- '---\n:https://git.teletypegames.org/api/packages/tools/rubygems: Bearer %s\n' "$${FORGE_TOKEN}" > ~/.gem/credentials
       - chmod 600 ~/.gem/credentials
       - gem push --host https://git.teletypegames.org/api/packages/tools/rubygems warp_engine-*.gem
     when: