ci: minimize schedule, safer push (#47)

* ci: only run mon-thu
* ci: don't error if remote branch already exists

.github/workflows/sync.yaml

@@ -3,7 +3,7 @@ name: Sync
 on:
   workflow_dispatch:
   schedule:
-    - cron: '0 0 * * *'
+    - cron: '0 0 * * 1-4'
 
 permissions:
   contents: write
@@ -47,6 +47,9 @@ jobs:
           branch="swagger/${api_server_revision}"
 
           set -x
+          # If remote branch exists, just terminate early.
+          git fetch origin "${branch}" && exit 0 || true
+
           git checkout -b "${branch}"
           git config --global user.email "[email protected]"
           git config --global user.name "FireHydrant Bot"