Fix nightly build pipeline.

Change-Id: I21eb66d204bb7e2a46c41ecc9a530f19516c88a4
diff --git a/buildkite/bazelci.py b/buildkite/bazelci.py
index 70551c5..35f0941 100644
--- a/buildkite/bazelci.py
+++ b/buildkite/bazelci.py
@@ -214,13 +214,13 @@
         "name": "Ubuntu 16.04",
         "emoji-name": ":ubuntu: 16.04",
         "agent-directory": "/var/lib/buildkite-agent/builds/${BUILDKITE_AGENT_NAME}",
-        "nightly": True
+        "nightly": False
     },
     "ubuntu1804": {
         "name": "Ubuntu 18.04",
         "emoji-name": ":ubuntu: 18.04",
         "agent-directory": "/var/lib/buildkite-agent/builds/${BUILDKITE_AGENT_NAME}",
-        "nightly": True
+        "nightly": False
     },
     "macos": {
         "name": "macOS",
@@ -1100,6 +1100,12 @@
     if not configs:
         raise BuildkiteException("Bazel publish binaries pipeline configuration is empty.")
 
+    for platform in configs.copy():
+        if not platform in PLATFORMS:
+            raise BuildkiteException("Unknown platform '{}'".format(platform))
+        if not PLATFORMS[platform]["nightly"]:
+            del configs[platform]
+
     if set(configs) != set(name for name, platform in PLATFORMS.items() if platform["nightly"]):
         raise BuildkiteException("Bazel publish binaries pipeline needs to build Bazel on all nightly-enabled platforms.")