Update validate-markdown.yaml

Suggested fix from https://chromium.googlesource.com/chromium/src/+/main/docs/security/apparmor-userns-restrictions.md#option-3_the-safest-way

Author: ralfhandl

.github/workflows/validate-markdown.yaml

@@ -29,4 +29,6 @@ jobs:
         run: npx --yes markdownlint-cli2 *.md
 
       - name: Check links in markdown files
-        run: npx --yes @umbrelladocs/linkspector check
+        run: |
+          export CHROME_DEVEL_SANDBOX=/opt/google/chrome/chrome-sandbox
+          npx --yes @umbrelladocs/linkspector check