Prevent Dependabot workflows for private mirrors

[desrosj]

Currently, the if conditionals for workflows use one of two approaches to limit when they run: Run if the repository is WordPress/wordpress-develop OR if the workflow is triggered by a pull_request event. This prevents workflows from running needlessly on forks and mirrors when branches are synced or updated through push events, but still allows contributors to open PRs back to their forks to test changes before submitting them back.

This can cause some issues with Dependabot, though. Dependabot is disabled by default for all forks. However, it is impossible to disable Dependabot for mirrors when a dependabot.yml file is present. This is especially problematic for private mirrors, which consume resources for organizations.

This switches the OR condition to run workflows if triggered by a pull_request AND the actor is not dependabot to prevent these workflows from running in this situation.

Trac ticket: https://core.trac.wordpress.org/ticket/62221

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[desrosj]

Merged in https://core.trac.wordpress.org/changeset/60080.