nightly build workflow with gha

[anithapriyanatarajan]

Changes

This pull request introduces a new automated nightly build workflow for the Tekton Dashboard using GitHub Actions.

Nightly Build Automation

• Added a new GitHub Actions workflow (.github/workflows/nightly-builds.yml) to automate nightly builds of the Tekton Dashboard.
• Updated documentation (tekton/README.md) to describe the new nightly build workflow, its schedule, and mentions reuse of existing pipelines.

Pipeline and Publish Definition Updates

• Updated tekton/release-pipeline.yaml and tekton/publish.yaml to use ghcr.io as the default image registry, clarified version tag descriptions to support nightly builds, and modernized Tekton API versions and parameter defaults for nightly/stable releases.

Kubernetes Resource Improvements

• Added tekton/account.yaml to define service accounts, secrets, RBAC roles, and bindings necessary for secure pipeline execution and resource access during nightly builds.

Submitter Checklist

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

• Docs included if any changes are user facing
• Tests included if any functionality added or changed
• 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 (new features, significant UI changes, API changes, bug fixes, changes requiring upgrade notices or deprecation warnings)
• Release notes contains the string "action required" if the change requires additional action from users switching to the new release

Release Notes

NONE

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[anithapriyanatarajan]

/kind misc

[anithapriyanatarajan]

/hold

[anithapriyanatarajan]

Kept on hold to retry the possibility of managing this as a composite workflow from plumbing repo. tektoncd/plumbing#2671

[AlanGreene]

Closing in favour of #4414