Use GITHUB_TOKEN provided by GitHub Actions in workflows

That's why we were seeing multiple comments.

See:

- danger/danger-js#856 (comment)
- https://help.github.com/en/actions/configuring-and-managing-workflows/authenticating-with-the-github_token

.github/workflows/danger-pr-labels.yml

@@ -13,4 +13,4 @@ jobs:
         with:
           args: "--dangerfile Automattic/peril-settings/org/pr/label.ts"
         env:
-          GITHUB_TOKEN: ${{ secrets.MOKAGIO_BOT_GITHUB_TOKEN }}
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}

.github/workflows/danger-prs.yml

@@ -15,4 +15,4 @@ jobs:
         with:
           args: "--dangerfile Automattic/peril-settings/org/pr/ios-macos.ts"
         env:
-          GITHUB_TOKEN: ${{ secrets.MOKAGIO_BOT_GITHUB_TOKEN }}
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}