CI: try dune-cache (#353)

.github/workflows/test.yml

@@ -28,6 +28,7 @@ jobs:
     - uses: ocaml/setup-ocaml@v3
       with:
         ocaml-compiler: ${{matrix.ocaml}}
+        dune-cache: true
 
     # For Caqti PostgreSQL examples. opam does actually install PostgreSQL for
     # us. However, Homebrew doesn't link it by default, so we have to install
@@ -94,6 +95,7 @@ jobs:
     - uses: ocaml/setup-ocaml@v3
       with:
         ocaml-compiler: ${{matrix.ocaml}}
+        dune-cache: true
 
     - name: Run quickstart.sh
       shell: bash
@@ -123,6 +125,7 @@ jobs:
     - uses: ocaml/setup-ocaml@v3
       with:
         ocaml-compiler: 4.14.x
+        dune-cache: true
     # Needed until https://github.com/robur-coop/ocaml-letsencrypt/pull/34.
     - run: opam pin add letsencrypt git+https://github.com/hannesm/ocaml-letsencrypt.git#no-cstruct --no-action
     - run: opam install --yes --deps-only ./dream-pure.opam ./dream-httpaf.opam ./dream.opam ./dream-mirage.opam