enable CI on 8.18

Author: gares

.github/workflows/main.yml

@@ -19,6 +19,7 @@ jobs:
         coq_version:
           - '8.16'
           - '8.17'
+          - '8.18'
     steps:
     - uses: actions/checkout@v2
     - uses: coq-community/docker-coq-action@v1