chore: add missing install command to new workflow

Author: SimenB

.github/workflows/markdown-link-check.yml

@@ -27,6 +27,8 @@ jobs:
         with:
           node-version: lts/*
           cache: yarn
+      - name: install
+        run: yarn
       - name: Run markdown-link-check on MD files
         run:
           find . -name "*.md" | grep -v node_modules | xargs -n 1 yarn