Use cluster issuer as default builder id

[wlynch]

Changes

This is probably the most "correct" value for this field - it should uniquely identify the cluster, and will match other signature data included in Fulcio certs, etc.

This is technically a breaking change, but likely one worth making. Users can still override this behavior with the config map as before. If omitted, this field is not populated as an indication that we don't know how to accurately identify this cluster.

Part of #640 - this doesn't resolve the SLSA Build Level aspects of the Builder ID (which I wish it didn't have to be encoded in the Builder ID, but that ship has sailed for SLSA v1 ¯_(ツ)_/¯)

Submitter Checklist

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

• Has Docs included if any changes are user facing
• Has Tests included if any functionality added or changed
• Follows the commit message standard
• Meets the Tekton contributor standards (including
  functionality, content, code)
• Release notes block below has been updated with any user facing 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

BREAKING CHANGE: The default Builder ID for SLSA provenance now defaults to the cluster's OIDC token issuer instead of a static string.

[tekton-robot]

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[tekton-robot]

The following is the coverage report on the affected files.
Say /test pull-tekton-chains-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/internal/cluster/builder_id.go	Do not exist	68.2%

[tekton-robot]

The following is the coverage report on the affected files.
Say /test pull-tekton-chains-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/internal/cluster/builder_id.go	Do not exist	68.2%

[tekton-robot]

@wlynch: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-tekton-chains-unit-tests	b50350b	link	true	/test pull-tekton-chains-unit-tests
pull-tekton-chains-integration-tests	b50350b	link	true	/test pull-tekton-chains-integration-tests

Full PR test history. Your PR dashboard.

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[tekton-robot]

@wlynch: PR needs rebase.

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