Exclude gnu.org and fsf.org from link checker (#454)

The link checker is timing out at gnu.org and fsf.org. Propose to
exclude as this is not the first time we've encountered this. And we
can't _remove_ the links: it's part of the licenses.

Would unblock @renovate bot's many updates.

Author: samcunliffe

.github/workflows/links.yml

@@ -23,11 +23,14 @@ jobs:
         with:
           # Exclude the image.sc forum (which works) but returns 403 from
           # github runners and exclude relative links to other pages. Ignore
-          # the whole of the project slug README since the links are
+          # gnu.org and fsf.org because they're often down but are still the right
+          # websites to link to!
+          #
+          # Ignore the whole of the project slug README since the links are
           # necessarily not real links - i.e. demos and to be filled by
           # cookiecutter values.
           args:
-            "--verbose --exclude https://forum.image.sc/ --exclude docs/pages
+            "--verbose --exclude https://forum.image.sc/ --exclude http://www.gnu.org --exclude http://fsf.org --exclude docs/pages
             --exclude-path '{{cookiecutter.project_slug}}/README.md'  -- ."
           fail: true
           jobSummary: true