trigger opam-update repo after successful CI build

.gitlab-ci.yml

 image: ralfjung/opam-ci:latest
 
+stages:
+  - build
+  - opam
+
 variables:
   CPU_CORES: "9"
 
 .template: &template
+  stage: build
   tags:
   - fp-timing
   script:
@@ -23,6 +28,11 @@ variables:
   - master
   - /^ci/
 
+opam:
+  stage: opam
+  script:
+  # Send a trigger to the repository doing the work
+  - curl --fail -X POST -F "token=$OPAM_UPDATE_SECRET" -F "ref=master" -F "variables[REPO]=${CI_PROJECT_URL}.git" -F "variables[REF]=$CI_COMMIT_REF_NAME" -F "variables[SHA]=$CI_COMMIT_SHA" -F "variables[NAME]=coq-stdpp" https://gitlab.mpi-sws.org/api/v3/projects/581/trigger/builds
 
 stdpp-coq8.7:
   <<: *template