[ci] opam cache buster

.github/workflows/main.yml

@@ -257,7 +257,7 @@ jobs:
         uses: actions/[email protected]
         with:
           path: ~/.opam/
-          key: ${{ runner.os }}-${{ hashFiles('./opam', './libs/') }}-1
+          key: ${{ runner.os }}-${{ hashFiles('./opam', './libs/') }}-2
 
       - name: Install Neko from S3
         run: |