fixup! OxCaml: setup CI

Author: vouillon

.github/workflows/wasm_of_ocaml.yml

@@ -138,8 +138,10 @@ jobs:
         run: opam pin add ppxlib -n 0.35.0
         if: matrix.ocaml-compiler != 'ocaml-variants.5.2.0+ox'
 
-      - name: Pin qcheck
-        run: opam pin add -n https://github.com/vouillon/qcheck.git#oxcaml
+      - name: Pin specific packages for OxCaml
+        run: |
+          opam pin add -n https://github.com/vouillon/qcheck.git#oxcaml
+          opam pin add -n zarith.1.12+ox
         if: matrix.ocaml-compiler == 'ocaml-variants.5.2.0+ox'
 
       - name: Checkout Jane Street packages