doc,tools: annotate broken links in actions workflow

Change format of logged messages when run on GitHub Actions (i.e.
when the `GITHUB_ACTIONS` environment variable is true) so that
broken links are highlighted inline in pull requests.

Signed-off-by: Richard Lau <riclau@uk.ibm.com>

tools/doc/checkLinks.js

@@ -50,7 +50,9 @@ function checkFile(path) {
     const targetURL = new URL(node.url, base);
     if (targetURL.protocol === 'file:' && !fs.existsSync(targetURL)) {
       const { line, column } = node.position.start;
-      console.error(`Broken link at ${path}:${line}:${column} (${node.url})`);
+      console.error((process.env.GITHUB_ACTIONS ?
+        `::error file=${path},line=${line},col=${column}::` : '') +
+        `Broken link at ${path}:${line}:${column} (${node.url})`);
       process.exitCode = 1;
     }
   }