CI pkg:opam:native fix test

https://docs.gitlab.com/ee/ci/yaml/#after_script

>Don’t affect the job’s exit code. If the script section succeeds and
the after_script times out or fails, the job exits with code 0 (Job
Succeeded).

.gitlab-ci.yml

@@ -230,11 +230,15 @@ before_script:
   extends: .auto-use-tags
   # OPAM will build out-of-tree so no point in importing artifacts
   script:
+    - if [ "$COQ_CI_NATIVE" = true ]; then opam install -y coq-native; fi
     - opam pin add --kind=path coq-core.dev .
     - opam pin add --kind=path rocq-core.dev .
     - opam pin add --kind=path coq-stdlib.dev stdlib/
     - opam pin add --kind=path coqide-server.dev .
     - opam pin add --kind=path coqide.dev .
+    - if [ "$COQ_CI_NATIVE" = true ]; then echo "Definition f x := x + x." > test_native.v; fi
+    - if [ "$COQ_CI_NATIVE" = true ]; then coqc test_native.v; fi
+    - if [ "$COQ_CI_NATIVE" = true ]; then test -f .coq-native/Ntest_native.cmxs; fi
   variables:
     OPAM_VARIANT: "+flambda"
   only: *full-ci
@@ -365,13 +369,8 @@ lint:
 
 pkg:opam:native:
   extends: .pkg:opam-template
-  before_script:
-    - opam install -y coq-native
-  after_script:
-    - eval $(opam env)
-    - echo "Definition f x := x + x." > test_native.v
-    - coqc -native-compiler yes test_native.v
-    - test -f .coq-native/Ntest_native.cmxs
+  variables:
+    COQ_CI_NATIVE: "true"
 
 # broken, see eg https://gitlab.com/coq/coq/-/jobs/1754045983
 # pkg:nix:deploy: