Merge pull request rust-lang#2296 from jieyouxu/no-external-linkcheck

Don't linkcheck external web links in PR CI

Author: jieyouxu

src/doc/rustc-dev-guide/.github/workflows/ci.yml

@@ -18,6 +18,7 @@ jobs:
       MDBOOK_LINKCHECK2_VERSION: 0.9.1
       MDBOOK_MERMAID_VERSION: 0.12.6
       MDBOOK_TOC_VERSION: 0.11.2
+      MDBOOK_OUTPUT__LINKCHECK__FOLLOW_WEB_LINKS: ${{ github.event_name != 'pull_request' }}
       DEPLOY_DIR: book/html
       BASE_SHA: ${{ github.event.pull_request.base.sha }}
       GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}