Add env. variable support

Author: marxin

Diff for: README.md

@@ -59,16 +59,21 @@ The build files are found in the `book/html` directory.
 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.
 
+> [!NOTE]
+> The link validation can be skipped by setting the following environment variable:
+> `SKIP_LINKCHECK=1 mdbook ...`
+
 ### Table of Contents
 
 We use `mdbook-toc` to auto-generate TOCs for long sections. You can invoke the preprocessor by
 including the `<!-- toc -->` marker at the place where you want the TOC.
 
 ## How to fix toolstate failures
 
-> **NOTE**: Currently, we do not track the rustc-dev-guide toolstate due to
-[spurious failures](https://github.com/rust-lang/rust/pull/71731),
-but we leave these instructions for when we do it again in the future.
+> [!NOTE]
+> Currently, we do not track the rustc-dev-guide toolstate due to
+> [spurious failures](https://github.com/rust-lang/rust/pull/71731),
+> but we leave these instructions for when we do it again in the future.
 
 1. You will get a ping from the toolstate commit. e.g. https://github.com/rust-lang-nursery/rust-toolstate/commit/8ffa0e4c30ac9ba8546b7046e5c4ccc2b96ebdd4
 

Diff for: ci/linkcheck.sh

@@ -7,6 +7,11 @@ set_github_token() {
   jq '.config.output.linkcheck."http-headers"."github\\.com" = ["Authorization: Bearer $GITHUB_TOKEN"]'
 }
 
+if [ ! -z "$SKIP_LINKCHECK" ] ; then
+  echo "Skipping link check."
+  exec mdbook-linkcheck -f ""
+fi
+
 # https://docs.github.com/en/actions/reference/environment-variables
 if [ "$GITHUB_EVENT_NAME" = "schedule" ] ; then # running in scheduled job
   FLAGS=""