ci: add vanilla opam build.

.github/workflows/opam.yml

@@ -0,0 +1,26 @@
+name: Build with opam
+
+on:
+  push:
+    paths: [ '*.opam' ]
+  pull_request:
+    paths: [ '*.opam' ]
+  workflow_dispatch:
+
+# only run one instance of this action at a time.
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v4
+    - uses: ocaml/setup-ocaml@v2
+      with:
+        ocaml-compiler: 4.09
+    - run: opam install . --deps-only --with-test
+    - run: dune build --profile release
+

.github/workflows/test.yml

@@ -28,12 +28,6 @@ jobs:
       with: { name: pac-nix }
     - run: echo 'preparing nix shell environment'
 
-    # it will be nice to make this work, just in case we aslp adds new dependencies 
-    # - uses: ocaml/setup-ocaml@v2
-    #   with:
-    #     ocaml-compiler: ocaml-system.4.09.1 # must match nix develop's ocaml version!
-    # - run: opam install . --deps-only --with-test
-
     - run: dune build --profile release
     - run: ./coverage.sh test
       id: coverage