Don't rebuild/retest release tags (#5385)

ci/buildkite-release.yml

@@ -0,0 +1,15 @@
+# Build steps that run on a release tag
+#
+# All the steps in `buildkite.yml` are skipped and we jump directly to the
+# secondary build steps since it's assumed the commit that was tagged is known
+# to be good so there's no need to rebuild and retest it.
+steps:
+  - trigger: "solana-secondary"
+    branches: "!pull/*"
+    async: true
+    build:
+      message: "${BUILDKITE_MESSAGE}"
+      commit: "${BUILDKITE_COMMIT}"
+      branch: "${BUILDKITE_BRANCH}"
+      env:
+        TRIGGERED_BUILDKITE_TAG: "${BUILDKITE_TAG}"

ci/buildkite-secondary.yml

@@ -1,3 +1,6 @@
+#
+# Build steps that run after the primary pipeline on pushes and tags.
+# Pull requests to not run these steps.
 steps:
   - command: "sdk/docker-solana/build.sh"
     timeout_in_minutes: 60

ci/buildkite.yml

@@ -1,3 +1,6 @@
+# Build steps that run on pushes and pull requests.
+#
+# Release tags use buildkite-release.yml instead
 steps:
   - command: "ci/shellcheck.sh"
     name: "shellcheck"