Don't allow unsupported compilers to be selected

Author: dra27

.github/workflows/windows.yml

@@ -40,6 +40,11 @@ jobs:
         if: steps.cache-opam.outputs.cache-hit != 'true'
         run: opam init --yes --no-setup .
 
+      - name: Restrict testing to available compilers
+        if: steps.cache-opam.outputs.cache-hit != 'true'
+        # TODO Amend this lowerbound as older compiler packages are updated
+        run: opam switch set-invariant --formula "`"ocaml`" {>= `"4.13`"}"
+
       - name: Save opam cache
         if: steps.cache-opam.outputs.cache-hit != 'true'
         uses: actions/cache/save@v4