Reduce number of BCR CI jobs

During queue times it can get annoying. Having macOS and any Linux
should be good enough for this repo, we're just validating that things
work at all with these.

.bcr/presubmit.yml

@@ -1,5 +1,5 @@
 matrix:
-  platform: ["centos7", "debian10", "macos", "ubuntu2004", "windows"]
+  platform: ["macos_arm64", "ubuntu2004"]
   bazel: ["6.x", "7.x", "rolling"]
 
 tasks: