Gitlab-CI: Explicitly tag specified arm32 images.

So we don't accidentally run generic images on runners
that only can deal with specific images.

.gitlab-ci.yml

@@ -228,7 +228,7 @@ arm:
   artifacts: *release-artifacts
   only: *release-only
   tags:
-    - arm32
+    - arm32-specified
   image: arm32v7/elixir:1.10.3
   cache: *release-cache
   variables: *release-variables
@@ -240,7 +240,7 @@ arm-musl:
   artifacts: *release-artifacts
   only: *release-only
   tags:
-    - arm32
+    - arm32-specified
   image: arm32v7/elixir:1.10.3-alpine
   cache: *release-cache
   variables: *release-variables