Prevent duplicate CI runs

[kiendang]

We have this in our workflows

on: [push, pull_request]

This is redundant since when someone pushes to a PR branch, it triggers both push and pull_request synchronized which makes the action runs twice. Checking every PR CI and you will see we have the every check run twice (push) and (pull_request).

Just on: push is enough.

[sjdemartini]

Good call!

[kiendang]

Just learned that the reason for on: [push, pull_request] is that push does not trigger for PRs from forks (graphql-python/graphql-server#117 (review)). I've fixed this, but put in a new PR #1443 from my fork so we can test.