move OPAM_PKG to the 8.8+beta1 job as that one is spawned first

.gitlab-ci.yml

@@ -43,6 +43,8 @@ build-coq.8.8+beta1:
   variables:
     OPAM_PINS: "coq version 8.8+beta1"
     VALIDATE: "1"
+    OPAM_PKG: "coq-iris"
+    OPAM_PKG_BRANCH: "gen_proofmode"
     TIMING_PROJECT: "iris"
     TIMING_CONF: "coq-8.8+beta1"
   tags:
@@ -52,8 +54,6 @@ build-coq.8.7.2:
   <<: *template
   variables:
     OPAM_PINS: "coq version 8.7.2"
-    OPAM_PKG: "coq-iris"
-    OPAM_PKG_BRANCH: "gen_proofmode"
     TIMING_PROJECT: "iris"
     TIMING_CONF: "coq-8.7.2"
   tags: