fix: try on: pull_request_target to share secrets with fork instead of on: pull_request

[anitarua]

on: pull_request_target allows the sharing of secrets with forks, and it should be used with a safeguard, such as checking the permissions of the triggering actor, as we do here.

on: pull_request will not share secrets with forks, even if triggering actor has write permissions, as seen with this PR: #6

Also did some minor cleanup of the workflow file:

• Determined that the default ref used by checkout action is fine, removed the second block with non-default ref
• Use if condition in github actions syntax to avoid printing out the secret when checking if it exists