Only run actions on push, not pull_request

[samredai]

Small change to not run actions on pull requests. I noticed one other issue as well where apache doesn't allow using the ciiiii/toml-editor@1.0.0 action.

Error : .github#L1
ciiiii/toml-editor@1.0.0, ciiiii/toml-editor@1.0.0, ciiiii/toml-editor@1.0.0, and ciiiii/toml-editor@1.0.0 are not allowed to be used in apache/iceberg-docs. Actions in this workflow must be: within a repository owned by apache, created by GitHub, verified in the GitHub Marketplace or match the following: /@[a-f0-9][a-f0-9][a-f0-9][a-f0-9][a-f0-9][a-f0-9][a-f0-9]+, AdoptOpenJDK/install-jdk@, JamesIves/github-pages-deploy-action@5dc1d5a, TobKed/label-when-approved-action@, actions-cool/issues-helper@, actions-rs/, al-cheb/configure-pagefile-action@, amannn/action-semantic-pull-request@, apache/, burrunan/gradle-cache-action@, bytedeco/javacpp-presets/.github/actions/, chromaui/action@, codecov/codecov-action@, conda-incubator/setup-miniconda@, container-tools/kind-action@, container-tools/microshift-action@, dawidd6/action-download-artifact@, delaguardo/setup-graalvm@, docker://pandoc/core:2.9, eps1lon/actions-label-merge-conflict@*,...

I'll look into that and provide an update in a follow-up PR.