Add rules_platform to downstream pipeline (#1879)

Fixes https://github.com/bazelbuild/continuous-integration/issues/1852
diff --git a/buildkite/bazelci.py b/buildkite/bazelci.py
index 58de26f..ac78bf2 100755
--- a/buildkite/bazelci.py
+++ b/buildkite/bazelci.py
@@ -340,6 +340,10 @@
         "git_repository": "https://github.com/bazelbuild/rules_perl.git",
         "pipeline_slug": "rules-perl",
     },
+    "rules_platform": {
+        "git_repository": "https://github.com/bazelbuild/rules_platform.git",
+        "pipeline_slug": "rules-platform",
+    },
     "rules_proto": {
         "git_repository": "https://github.com/bazelbuild/rules_proto.git",
         "pipeline_slug": "rules-proto",