ci: use prerelease opam version to package releases

This is opam 2.3.0~alpha1

Signed-off-by: Pau Ruiz Safont <pau.ruizsafont@cloud.com>

.github/workflows/release.yml

@@ -21,6 +21,7 @@ jobs:
           xs-opam: "."
         opam-pin: false
         dune-cache: true
+        allow-prerelease-opam: true
 
     - name: Collect licenses
       run: ./tools/print-licenses.sh > licenses.txt