ci: drop s390x build, rename steps to amd64/arm64

s390x had no dedicated builder and ran as a cross-compile pinned to
amd64 via nodeSelector, colliding with the x86_64-linux step on the
same node — Woodpecker's k8s backend couldn't create the per-step
secret twice and the workflow failed with either "secrets already
exists" or "Canceled". Disable until a real s390x builder is wired up.

Step names now match the kubernetes.io/arch label they target; the
ci/build.py argument keeps the Nix system tuple (x86_64-linux,
aarch64-linux).

.woodpecker.yml

@@ -17,7 +17,7 @@ clone:
 
 # Build packages for each architecture in parallel
 steps:
-  - name: build-x86_64-linux
+  - name: build-amd64
     image: git.oleks.space/oleks/nix-ci:latest
     depends_on: []
     environment:
@@ -37,7 +37,7 @@ steps:
       - sh ci/setup.sh
       - python3 ci/build.py x86_64-linux
 
-  - name: build-aarch64-linux
+  - name: build-arm64
     image: git.oleks.space/oleks/nix-ci:latest
     depends_on: []
     environment:
@@ -57,22 +57,23 @@ steps:
       - sh ci/setup.sh
       - python3 ci/build.py aarch64-linux
 
-  - name: build-s390x-linux
-    image: git.oleks.space/oleks/nix-ci:latest
-    depends_on: []
-    environment:
-      ATTIC_TOKEN:
-        from_secret: attic_token
-      GITEA_CLONE_TOKEN:
-        from_secret: gitea_clone_token
-    backend_options:
-      kubernetes:
-        nodeSelector:
-          kubernetes.io/arch: amd64
-        labels:
-          commit-tag: "${CI_COMMIT_TAG}"
-          commit-branch: "${CI_COMMIT_BRANCH}"
-          pipeline-number: "${CI_PIPELINE_NUMBER}"
-    commands:
-      - sh ci/setup.sh
-      - python3 ci/build.py s390x-linux
+  # s390x disabled: no builder available
+  # - name: build-s390x
+  #   image: git.oleks.space/oleks/nix-ci:latest
+  #   depends_on: []
+  #   environment:
+  #     ATTIC_TOKEN:
+  #       from_secret: attic_token
+  #     GITEA_CLONE_TOKEN:
+  #       from_secret: gitea_clone_token
+  #   backend_options:
+  #     kubernetes:
+  #       nodeSelector:
+  #         kubernetes.io/arch: amd64
+  #       labels:
+  #         commit-tag: "${CI_COMMIT_TAG}"
+  #         commit-branch: "${CI_COMMIT_BRANCH}"
+  #         pipeline-number: "${CI_PIPELINE_NUMBER}"
+  #   commands:
+  #     - sh ci/setup.sh
+  #     - python3 ci/build.py s390x-linux