update the Coq version we run nightly

.gitlab-ci.yml

@@ -54,7 +54,7 @@ build-coq.8.7.2:
 build-iris.dev:
   <<: *template
   variables:
-    OPAM_PINS: "coq version 8.8.2   coq-stdpp.dev git git+https://gitlab.mpi-sws.org/iris/stdpp.git#$STDPP_REV   coq-iris.dev git git+https://gitlab.mpi-sws.org/FP/iris-coq.git#$IRIS_REV"
+    OPAM_PINS: "coq version 8.9.0   coq-stdpp.dev git git+https://gitlab.mpi-sws.org/iris/stdpp.git#$STDPP_REV   coq-iris.dev git git+https://gitlab.mpi-sws.org/FP/iris-coq.git#$IRIS_REV"
   except:
   only:
   - triggers