CI: fix action syntax

Author: bsipocz

.github/workflows/milestone.yml

@@ -15,4 +15,4 @@ jobs:
     steps:
       - uses: scientific-python/attach-next-milestone-action@a4889cfde7d2578c1bc7400480d93910d2dd34f6
         env:
-          GITHUB_TOKEN:: ${{ secrets.GITHUB_TOKEN }}
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}