CI: pin zarith, fix windows failure

Author: hhugo

.github/workflows/build.yml

@@ -101,6 +101,9 @@ jobs:
           opam-depext: ${{ !matrix.skip-test }}
           opam-depext-flags: --with-test
 
+      - run: opam pin add zarith git+https://github.com/dra27/Zarith.git#windows-cc
+        if: runner.os == 'Windows'
+
       - run: opam install . --best-effort
         if: ${{ matrix.skip-test }}