ci: echo build arch (uname -m) as first line of every step for visibility

.woodpecker/amd64.yaml

@@ -35,6 +35,7 @@ steps:
           commit-branch: "${CI_COMMIT_BRANCH}"
           pipeline-number: "${CI_PIPELINE_NUMBER}"
     commands:
+      - echo "▸ arch=$(uname -m)"
       - sh ci/setup.sh
       # Same entrypoint as a local `nix run .#publish-amd64 -- --push`.
       # PUBLISH=1 makes the shared script actually push (local runs dry-run).

.woodpecker/arm64.yaml

@@ -33,6 +33,7 @@ steps:
           commit-branch: "${CI_COMMIT_BRANCH}"
           pipeline-number: "${CI_PIPELINE_NUMBER}"
     commands:
+      - echo "▸ arch=$(uname -m)"
       - sh ci/setup.sh
       # NODE-BOUND LEG (emmett#44, cluster#192): aarch64-linux can't be built on
       # emmett (linux/amd64) and these native packages have no cross path, so this