Test whether setup-ocaml works at all on the ARM64 runner

Author: cknitt

.github/workflows/ci.yml

@@ -160,6 +160,8 @@ jobs:
             ocaml_compiler: ocaml-variants.4.14.2+options,ocaml-option-static
           - os: ubuntu-latest
             ocaml_compiler: ocaml-variants.4.13.0+options,ocaml-option-static
+          - os: buildjet-2vcpu-ubuntu-2204-arm # ARM
+            ocaml_compiler: ocaml-variants.5.2.0+options,ocaml-option-static
 
     runs-on: ${{matrix.os}}
 
@@ -224,6 +226,8 @@ jobs:
         uses: ocaml/[email protected]
         if: steps.cache-opam-env.outputs.cache-hit != 'true'
         with:
+          opam-repositories: |
+            default: https://github.com/cknitt/opam-repository.git
           ocaml-compiler: ${{matrix.ocaml_compiler}}
           opam-pin: false