fix: 8.11 is not a float

Author: erikmd

.github/workflows/build-coq-demo.yml

@@ -23,7 +23,7 @@ jobs:
     strategy:
       matrix:
         coq_version:
-          - 8.11
+          - '8.11'
           - dev
         ocaml_version: ['4.07-flambda']
       # at most 20 concurrent jobs per free account: