Fix logging when no Bazel version is set (#2310)

Example failure:
https://buildkite.com/bazel-trusted/publish-bazel-binaries/builds/25622#0196edc4-5903-40e2-b921-9d3d1dc4b379
diff --git a/buildkite/bazelci.py b/buildkite/bazelci.py
index 145ea5b..994f2cf 100755
--- a/buildkite/bazelci.py
+++ b/buildkite/bazelci.py
@@ -1171,9 +1171,12 @@
         os.environ["USE_BAZEL_VERSION"] = download_bazel_binary(tmpdir, binary_platform)
         print_collapsed_group(":bazel: Using Bazel at " + os.environ["USE_BAZEL_VERSION"])
     else:
-        print_collapsed_group(":bazel: Using Bazel version " + bazel_version)
         if bazel_version:
+            print_collapsed_group(f":bazel: Using Bazel version {bazel_version}")
             os.environ["USE_BAZEL_VERSION"] = bazel_version
+        else:
+            print_collapsed_group(":bazel: Using latest Bazel release")
+
     if "USE_BAZEL_VERSION" in os.environ and not task_config.get(
         "skip_use_bazel_version_for_test", False
     ):