support/misc/gitlab-ci.yml.in: pass emulator builtin binaries as artifacts

Notes: We can't use runtime_test_download job from the parent pipeline
(generate-gitlab-ci) since the artifacts archive size is limited to 5MB.
So introduce a new custom stage named "download" executed before "test"
stage. test-dl directory that contain downloaded files can be an
artifact of the job passed to all jobs of next stages.

Fixes:
https://gitlab.com/buildroot.org/buildroot/-/jobs/4409032417

Runtime tested:
https://gitlab.com/kubu93/buildroot/-/pipelines/934319226

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

support/scripts/generate-gitlab-ci-yml

@@ -132,6 +132,7 @@ gen_tests() {
     fi
 
     if ${do_runtime:-false}; then
+        printf 'runtime_test_download: { extends: .runtime_test_download }\n'
         printf '%s: { extends: .runtime_test_base }\n' "${runtimes[@]}"
     fi