Fix bcr presubmit flags (#1151)

I accidentally used this as a target

Author: keith

.bcr/presubmit.yml

@@ -26,4 +26,5 @@ tasks:
     platform: macos
     build_targets:
     - "@rules_swift//examples/apple/..."
+    build_flags:
     - "--repo_env=BAZEL_DO_NOT_DETECT_CPP_TOOLCHAIN=1"