ci: milestone workflow should use our vouch app token

This increases our rate limits and the vouch app already has the
permissions required for the milestone workflow.

.github/workflows/milestone.yml

@@ -27,8 +27,7 @@ jobs:
         if: github.event.pull_request.merged == true
         with:
           action: bind-pr # `bind-pr` is the default action
-        env:
-          GITHUB_TOKEN: ${{ steps.app-token.outputs.token }}
+          github-token: ${{ steps.app-token.outputs.token }}
 
       # Bind milestone to closed issue that has a merged PR fix
       - name: Set Milestone for Issue
@@ -36,5 +35,4 @@ jobs:
         if: github.event.issue.state == 'closed'
         with:
           action: bind-issue
-        env:
-          GITHUB_TOKEN: ${{ steps.app-token.outputs.token }}
+          github-token: ${{ steps.app-token.outputs.token }}