Only build on `push`, not on `pull_request` too

[sambostock]

Listening for both events triggers a redundant build. All PRs will have involved pushing a branch, so the build will have already been triggered. There is no need to trigger it again.

[lugray]

This is a public repo. For external contributors, their pushes don't go to this repo, they go to their own clone, and so don't trigger CI. Once they open a PR on our repo, the PR triggers CI. For internal developers, builds on push are convenient to see failures before opening a PR.

[dylanahsmith]

Here is how I accomplished this in the public identity_cache repo: https://github.com/Shopify/identity_cache/commit/4a55be5b81c4190d3aeb85534ed44977a05df912