check if opam's there

Author: anmonteiro

.github/workflows/opam-build.yml

@@ -50,6 +50,11 @@ jobs:
         with:
           ocaml-compiler: ${{ matrix.ocaml-compiler }}
 
+      - name: Install dependencies (Windows)
+        if: ${{ matrix.os == 'windows-latest' }}
+        working-directory: melange
+        run: opam update
+
       - name: Install dependencies
         working-directory: melange
         run: |