Prevent duplicate GitHub Actions runs

When creating a PR, the GitHub Actions run twice due to the following definitions:

https://github.com/Automattic/hidden-posts/blob/9ecaf7aeaa076364bc042b46f53f480d6015cd18/.github/workflows/cs-lint.yml#L3-L9

https://github.com/Automattic/hidden-posts/blob/9ecaf7aeaa076364bc042b46f53f480d6015cd18/.github/workflows/integrations.yml#L3-L9

For example, the checks on https://github.com/Automattic/hidden-posts/pull/37#partial-pull-merging show these duplicate runs:

While GitHub Actions runs are free for public repos, we should use them mindful. Running them once both saves recourses and deliver faster results of the checks. Therefore, I propose running the GitHub Actions only on `push` and not on `pull_request`.