Consolidate the CI workflows into a single workflow

[AlanGreene]

Changes

Combine the E2E and Tekton PR workflows into a single Tekton CI workflow, and include the ability to trigger them manually.

Remove the push trigger as it's not necessary. The tests are run on each PR and can be run on demand if desired or as part of the release pipeline.

Example of how they're reported in the PR status:

Example of how they appear in the Actions view:

/kind misc

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

[briangleeson]

/lgtm

[tekton-robot]

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