Opt into, rather than out of, linkcheck (#2180)

This makes it less of a hassle to render the book locally.

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

@@ -57,7 +57,7 @@ jobs:
           cargo install mdbook-mermaid --version ${{ env.MDBOOK_MERMAID_VERSION }}
 
       - name: Check build
-        run: mdbook build
+        run: ENABLE_LINKCHECK=1 mdbook build
 
       - name: Deploy to gh-pages
         if: github.event_name == 'push'

src/doc/rustc-dev-guide/README.md

@@ -56,12 +56,14 @@ The build files are found in the `book/html` directory.
 
 ### Link Validations
 
-We use `mdbook-linkcheck2` to validate URLs included in our documentation.
-`linkcheck` will be run automatically when you build with the instructions in the section above.
+We use `mdbook-linkcheck2` to validate URLs included in our documentation. Link
+checking is **not** run by default locally, though it is in CI. To enable it
+locally, set the environment variable `ENABLE_LINKCHECK=1` like in the
+following example.
 
-> [!NOTE]
-> The link validation can be skipped by setting the following environment variable:
-> `SKIP_LINKCHECK=1 mdbook ...`
+```console
+$ ENABLE_LINKCHECK=1 mdbook serve
+```
 
 ### Table of Contents
 

src/doc/rustc-dev-guide/ci/linkcheck.sh

@@ -9,7 +9,7 @@ set_github_token() {
   jq '.config.output.linkcheck."http-headers"."github\\.com" = ["Authorization: Bearer $GITHUB_TOKEN"]'
 }
 
-if [ ! -z "$SKIP_LINKCHECK" ] ; then
+if [ -z "$ENABLE_LINKCHECK" ] ; then
   echo "Skipping link check."
   exit 0
 fi