build: fix external repo docs workflow

The external repo has the main branch as the default branch. This CL
adds it to the docs workflow trigger so the docs can be published
correctly.

GitOrigin-RevId: c55fc969b513663d9459c06ea77183cb9d1dbc96
diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index eb687b7..e9fe052 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -3,6 +3,7 @@
   push:
     branches:
       - master
+      - main
     paths:
       - 'docs/**'