Try to fix depexts in GitHub workflow

Author: mmottl

.github/workflows/main.yml

@@ -28,6 +28,9 @@ jobs:
         with:
           ocaml-compiler: 5
 
+      - name: Set PKG_CONFIG_PATH for libpq (keg-only)
+        run: echo "PKG_CONFIG_PATH=/opt/homebrew/opt/libpq/lib/pkgconfig" >> $GITHUB_ENV
+
       - run: opam install . --deps-only --with-test
 
       - run: opam exec -- dune build