split out 'make validate' into individual job

There's no need to run this for every compiler version; we only care
about the "main" version.

.gitlab-ci.yml

-# Taken from https://gitlab.com/erikmd/docker-coq-gitlab-ci-demo-1/blob/master/.gitlab-ci.yml
-
 .build:
   stage: build
   image: mathcomp/mathcomp:${CI_JOB_NAME}
   script:
     - ./create_makefile.sh
     - make -j ${NJOBS}
-    - make validate
 
 1.8.0-coq-8.8:
   extends: .build
 
-1.9.0-coq-8.9:
-  extends: .build
-
 1.9.0-coq-dev:
   extends: .build
   # it's ok to fail with an unreleased version of Coq
   allow_failure: true 
+
+1.9.0-coq-8.9:
+  extends: .build
+  # Keep track of all compiled output and the build infrastructure
+  artifacts:
+    name: build-files
+    paths:
+    - _CoqProject
+    - Makefile
+    - Makefile.conf
+    # Ugly hack around https://gitlab.com/gitlab-org/gitlab-runner/issues/2620
+    - "*/*.vo"
+    - "*/*/*.vo"
+    - "*/*/*/*.vo"
+    - "*/*/*/*/*.vo"
+    - "*/*/*/*/*/*.vo"
+    - "*/*/*/*/*/*/*.vo"
+    - "*/*/*/*/*/*/*/*.vo"
+    - "*/*/*/*/*/*/*/*/*.vo"
+    - "*/*/*/*/*/*/*/*/*/*.vo"
+    - "*/*.glob"
+    - "*/*/*.glob"
+    - "*/*/*/*.glob"
+    - "*/*/*/*/*.glob"
+    - "*/*/*/*/*/*.glob"
+    - "*/*/*/*/*/*/*.glob"
+    - "*/*/*/*/*/*/*/*.glob"
+    - "*/*/*/*/*/*/*/*/*.glob"
+    - "*/*/*/*/*/*/*/*/*/*.glob"
+    expire_in: 1 week
+   
+validate:
+  stage: test
+  image: mathcomp/mathcomp:1.9.0-coq-8.9
+  dependencies:
+    - 1.9.0-coq-8.9
+  script: make validate