Rework buildkite pipeline construction to be more composable

ci/test-sanity.sh

@@ -0,0 +1,14 @@
+#!/usr/bin/env bash
+set -e
+
+cd "$(dirname "$0")/.."
+
+source ci/_
+
+# Look for failed mergify.io backports
+_ git show HEAD --check --oneline
+
+_ ci/nits.sh
+_ ci/check-ssh-keys.sh
+
+echo --- ok