gitlab-ci: Explicitly list debian-10 in needs: for .deqp-test template

Apparently needs: in a definition overwrites inherited ones. So
.deqp-test effectively didn't declare needs: for debian-10, which means
any jobs based on .deqp-test could spuriously run after the debian-10
job failed or was cancelled.
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index ee611fd..6ee6e02 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -408,7 +408,9 @@
       - results/
   dependencies:
     - meson-main
-  needs: [meson-main]
+  needs:
+    - meson-main
+    - debian-10
 
 test-llvmpipe-gles2:
   parallel: 4