fix(ci): disable opam depext for ocamlformat

Otherwise it fails because it doesn't about the 'xapi-stdext' packge
(it is not part of xs-opam).

But ocamlformat shouldn't need depexts anyway, so disable them.

Signed-off-by: Edwin Török <[email protected]>

.github/workflows/format.yml

@@ -35,6 +35,7 @@ jobs:
             xs-opam: ${{ steps.dotenv.outputs.repository }}
           dune-cache: true
           opam-pin: false
+          opam-depext: false
 
       - name: Install ocamlformat
         run: opam install ocamlformat