gitlab-ci: fixup debian tags

When resolving a merge-conflict, I accidentally only updated the
ARM64-tag tag. Let's correct this.

Fixes: 3d529c1739 ("gitlab-ci: also build Zink on CI")

.gitlab-ci.yml

@@ -14,7 +14,7 @@
 # repository's registry will be used there as well.
 variables:
   UPSTREAM_REPO: mesa/mesa
-  DEBIAN_TAG: "2019-10-22"
+  DEBIAN_TAG: "2019-10-23"
   DEBIAN_ARM64_TAG: "arm64v8-2019-10-23"
   STRETCH_TAG: "2019-09-18"
   DEBIAN_VERSION: buster-slim