Remove Mac OS testing from the Bors merge flow.

It takes over an hour to run. It's unlikely to break if the main CI (Linux) passes, so running it during code review should be enough.

I also renamed it from mac_os to mac-os to be more consistent with size-diff and tock/tock's workflows.
diff --git a/.github/workflows/mac_os.yml b/.github/workflows/mac-os.yml
similarity index 78%
rename from .github/workflows/mac_os.yml
rename to .github/workflows/mac-os.yml
index 79bb1f2..c813ace 100644
--- a/.github/workflows/mac_os.yml
+++ b/.github/workflows/mac-os.yml
@@ -2,14 +2,9 @@
 
 name: ci-mac-os
 
-# We want to run CI on all pull requests. Additionally, Bors needs workflows to
-# run on the `staging` and `trying` branches.
-on:
-  pull_request:
-  push:
-    branches:
-      - staging
-      - trying
+# We run this workflow during pull request review, but not for Bors merges, as
+# it takes over an hour to run.
+on: pull_request
 
 jobs:
   ci-mac-os:
diff --git a/bors.toml b/bors.toml
index dfae601..4b9b851 100644
--- a/bors.toml
+++ b/bors.toml
@@ -1,5 +1,4 @@
 status = [
   "ci",
-  "ci-mac-os",
   "size-diff",
 ]