Use github runners instead of self-hosted

.github/workflows/build.yml

@@ -14,9 +14,10 @@ jobs:
       fail-fast: false
       matrix:
         os:
-          - self-hosted
+          - ubuntu-latest
         ocaml-compiler:
           - "4.14"
+          - "5.1"
     runs-on: ${{ matrix.os }}
     steps:
       - name: Checkout
@@ -27,6 +28,8 @@ jobs:
         uses: ocaml/setup-ocaml@v2
         with:
           ocaml-compiler: ${{ matrix.ocaml-compiler }}
+          dune-cache: true
+          allow-prerelease-opam: true
       - name: Install dependencies
         run: opam install -y . --deps-only --with-test
       - name: Build