chore: Move s390x build to post-merge.

It takes 15+ minutes and will very rarely fail, so it's not worth
blocking PRs on it.

.github/workflows/ci.yml

@@ -90,16 +90,6 @@ jobs:
         with:
           file: other/docker/compcert/Dockerfile
 
-  build-alpine-s390x:
-    runs-on: ubuntu-latest
-    steps:
-      - name: Set up Docker Buildx
-        uses: docker/setup-buildx-action@v1
-      - name: Docker Build
-        uses: docker/build-push-action@v2
-        with:
-          file: other/docker/alpine-s390x/Dockerfile
-
   cimplefmt:
     runs-on: ubuntu-latest
     steps:

.github/workflows/post-submit.yml

@@ -0,0 +1,16 @@
+name: post-submit
+
+on:
+  push:
+    branches: [master]
+
+jobs:
+  build-alpine-s390x:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v1
+      - name: Docker Build
+        uses: docker/build-push-action@v2
+        with:
+          file: other/docker/alpine-s390x/Dockerfile