Use opam install --update-invariant in lower-bounds-downgrade CI

Author: sim642

.github/workflows/ci.yml

@@ -83,7 +83,7 @@ jobs:
       - name: Downgrade dependencies
         # must specify ocaml-base-compiler again to prevent it from being downgraded
         # run: opam install $(opam exec -- opam-0install --prefer-oldest odep ocaml-base-compiler.${{ matrix.ocaml-compiler }})
-        run: opam install --unlock-base $(opam exec -- opam-0install --prefer-oldest --with-test odep)
+        run: opam install --update-invariant $(opam exec -- opam-0install --prefer-oldest --with-test odep)
 
       - name: Build
         run: opam exec -- dune build