Allow bench workflow to run on PRs from forks

[ashkrisk]

Allows all CI workflows to run correctly on PRs from forks. This mainly applies to the workflow which runs AutoBenchYAML.

• The workflow replaces github.head.ref (fork-workflow-2) with github.ref (/refs/pull/688/merge). The checkout action fails for the former if the branch is from a fork, but the latter works in both cases.
• The PR checklist commet workflow is updated to use the pull_request_target workflow rather the pull_request workflow. This allows the bot to add the PR checklist to PRs from forks. You'll notice that the bot did not comment on this PR, despite this change; this is because pull_request_target workflows are executed based on definitions in the main branch rather than the PR branch (for security reasons). This should work correctly on fork PRs raised after this PR is merged.

[ashkrisk]

(Manually adding the PR checklist for now)

Before you submit for review:

• Does your PR follow guidelines from CONTRIBUTIONS.md?
• Did you summarize what this PR does clearly and concisely?
• (NA) Did you include performance data for changes which may be performance impacting?
• (NA) Did you include useful docs for any user-facing changes or features?
• (NA) Did you include useful javadocs for developer oriented changes, explaining new concepts or key changes?
• Did you rebase your branch onto the latest main for regression testing and PR submission?
• Did you trigger regression testing via Run Bench Main and review results?
• Did you adhere to the code formatting guidelines (TBD)
• Did you group your changes for easy review, providing meaningful descriptions for each commit?
• Did you ensure that all files contain the correct copyright header?
• Did you add documentation for this feature to the release notes directory?

If you did not complete any of these, then please explain below.

• No release note is added since there are no user facing changes (except for contributors, but for them it's more like a bug fix).

[ashkrisk]

The default bench dataset is changed back to dpr-gemma-1m now that it's available in the public catalog.