Fix dune command

Use `dune exec` rather than `dune run`

Author: nickrobison

.github/workflows/fuzz.yml

@@ -22,4 +22,4 @@ jobs:
       - run: opam depext libmacaroons 
       - run: opam install . --deps-only
       - run: opam install . -y bun crowbar base64
-      - run: opam exec -- dune run libmacaroons/fuzz/libmacaroons_fuzz.exe
+      - run: opam exec -- dune exec libmacaroons/fuzz/libmacaroons_fuzz.exe