[sdk] Fix BuildArchitectures on macOS

Change-Id: Ieb1a0047d8edeadf5f24b058bf8828888412f00c
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/202763
Reviewed-by: Karl Klose <karlklose@google.com>
diff --git a/tools/bots/dart_sdk.py b/tools/bots/dart_sdk.py
index 67f983a..4625472 100755
--- a/tools/bots/dart_sdk.py
+++ b/tools/bots/dart_sdk.py
@@ -24,7 +24,7 @@
     if BUILD_OS == 'linux':
         return ['ia32', 'x64', 'arm', 'arm64']
     elif BUILD_OS == 'macos':
-        return BUILD_ARCHITECTURE
+        return [BUILD_ARCHITECTURE]
     else:
         return ['ia32', 'x64']