Increment opam cache key

Author: cknitt

.github/workflows/ci.yml

@@ -151,7 +151,7 @@ jobs:
       # matrix.ocaml_compiler may contain commas
       - name: Get OPAM cache key
         shell: bash
-        run: echo "opam_cache_key=opam-env-v3-${{ matrix.os }}-${{ matrix.ocaml_compiler }}-${{ hashFiles('dune-project') }}" | sed 's/,/-/g' >> $GITHUB_ENV
+        run: echo "opam_cache_key=opam-env-v4-${{ matrix.os }}-${{ matrix.ocaml_compiler }}-${{ hashFiles('dune-project') }}" | sed 's/,/-/g' >> $GITHUB_ENV
 
       - name: Restore OPAM environment
         id: cache-opam-env