[chore] Skip k6 twitter from linkchecker

It's been returning 403s

Author: MattDodsonEnglish

.github/workflows/staging.yml

@@ -139,6 +139,7 @@ jobs:
             https://linkedin.com,
             googleapis.com,
             https://www.linkedin.com/company/k6io,
-            https://staging.k6.io/slack
+            https://staging.k6.io/slack,
+            https://twitter.com/k6_io
           verbosity: "ERROR"
           markdown: false