chore(ci): include retest in slash workflow

[anithapriyanatarajan]

Changes

This pull request updates the GitHub Actions workflow for slash command routing to add support for a new /land command and includes some minor formatting improvements. The most important changes are:

Slash Command Support:

• Added support for the /land command, which will invoke the land-command workflow to land (merge) PRs stacked through ghstack. This expands the set of supported slash commands for repository automation.
• Added a configuration entry for the /retest command, granting write permission for pull requests in the tektoncd/pruner repository.

Formatting and Maintenance:

• Improved formatting in the YAML workflow for better readability, including expanding the types array and renaming steps for clarity.

Submitter Checklist

As the author of this PR, please check off the items in this checklist:

• Has Docs if any changes are user facing, including updates to minimum requirements e.g. Kubernetes version bumps
• Has Tests included if any functionality added or changed
• pre-commit Passed
• Follows the commit message standard
• Meets the Tekton contributor standards (including functionality, content, code)
• Has a kind label. You can add one by adding a comment on this PR that contains /kind <type>. Valid types are bug, cleanup, design, documentation, feature, flake, misc, question, tep
• Release notes block below has been updated with any user facing changes (API changes, bug fixes, changes requiring upgrade notices or deprecation warnings). See some examples of good release notes.
• Release notes contains the string "action required" if the change requires additional action from users switching to the new release

Release Notes

NONE

/kind misc

[infernus01]

/approve
/lgtm

[tekton-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: infernus01

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