workflow: run CI on pushes to main branch, not other branches (tmc#399)

* workflow: fix publish-docs workflow error (tmc#398)

Fix errors like https://github.com/tmc/langchaingo/actions/runs/7094309081

* workflow: run CI on pushes to main branch, not other branches

Right now, each PR runs CI twice - once because it's a PR, and another
time because it comes from a push to a branch that created the PR.
See for example tmc#398, causing:

- https://github.com/tmc/langchaingo/actions/runs/7094374726
- https://github.com/tmc/langchaingo/actions/runs/7094367361

This change makes it so we only run CI once per PR, because there's no
need to run it on a branch push.

Now CI runs on either a PR or a push to `main`

.github/workflows/ci.yaml

@@ -2,6 +2,8 @@ name: ci
 
 on:
   push:
+    branches:
+      - main
   pull_request:
     branches:
       - main