ci: lint_links: pin actions/checkout to v3

Author: elopez

.github/workflows/lint_links.yml

@@ -19,7 +19,7 @@ jobs:
   markdown-link-check:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@master
+      - uses: actions/checkout@v3
       - uses: gaurav-nelson/github-action-markdown-link-check@v1
         with:
           use-quiet-mode: "yes"