Define the Bikeshed build to fail on warnings

See https://github.com/w3c/spec-prod/blob/main/docs/options.md#build_fail_on

Fix #468

Author: anssiko

.github/workflows/auto-publish.yml

@@ -17,7 +17,7 @@ jobs:
           GH_PAGES_BRANCH: gh-pages
           TOOLCHAIN: bikeshed
           VALIDATE_LINKS: false
-          BUILD_FAIL_ON: link-error
+          BUILD_FAIL_ON: warning
           W3C_WG_DECISION_URL: https://lists.w3.org/Archives/Public/public-webmachinelearning-wg/2021Jun/0011.html
           W3C_ECHIDNA_TOKEN: ${{ secrets.ECHIDNA_TOKEN }}
           W3C_BUILD_OVERRIDE: |