fix dune in yml

Author: craff

.github/workflows/main.yml

@@ -36,5 +36,5 @@ jobs:
           architecture: ${{ matrix.arch }}
 
       - run: opam install . --deps-only --with-test -y
-      - run: dune build
-      - run: dune runtest
+      - run: opam exec -- dune build
+      - run: opam dune -- runtest