workflows/apidoc: try to rebase before pushing.

This avoids failures when you have two `master` builds running at once.

.github/workflows/apidoc.yml

@@ -37,5 +37,7 @@ jobs:
 
           if ! git diff --exit-code HEAD -- docs; then
             git commit -m 'docs: update from Homebrew/brew push' docs
+            git fetch
+            git rebase origin/master
             git push
           fi