ci: install ocamlformat separately due to removal from opam file

Author: yasunariw

.github/workflows/test.yml

@@ -35,6 +35,8 @@ jobs:
 
       - run: opam install . --deps-only --with-doc --with-test
 
+      - run: opam install ocamlformat.0.19.0
+
       - name: compile
         run: opam exec -- make