chore: use readonly deploy key for preview (#4645)

Author: jawnsy

.github/workflows/docs-preview.yaml

@@ -30,7 +30,7 @@ jobs:
         with:
           repository: coder/m
           ref: refs/heads/master
-          token: ${{ secrets.GH_ACCESS_TOKEN }}
+          ssh-key: ${{ secrets.READONLY_M_DEPLOY_KEY }}
           submodules: true
           fetch-depth: 0