Update merlin version in CI

Author: voodoos

.github/workflows/build-and-test.yml

@@ -51,8 +51,8 @@ jobs:
           ocaml-compiler: "5.2"
 
       # Remove this pin once a compatible version of Merlin has been released
-      # - name: Pin dev Merlin
-      #   run: opam pin https://github.com/ocaml/merlin.git#main
+      - name: Pin dev Merlin
+        run: opam pin https://github.com/voodoos/merlin.git#414-4.19-backports
 
       - name: Build and install dependencies
         run: opam install .