Disable builtin C++ toolchain in BCR presubmit (#1146)

Now that the BCR uses bzlmod the builtin toolchain is preferred which
doesn't work for us. This wouldn't be an issue if the tests in the bcr
ran within this repo

Author: keith

.bcr/presubmit.yml

@@ -26,3 +26,4 @@ tasks:
     platform: macos
     build_targets:
     - "@rules_swift//examples/apple/..."
+    - "--repo_env=BAZEL_DO_NOT_DETECT_CPP_TOOLCHAIN=1"