Add GitHub Actions workflow for go coverage job

[AlanGreene]

Changes

tektoncd/plumbing#2842

It uses workflow artifacts to store current coverage on merge to main, and uses this as a baseline for comparison when running on PRs.

It's also configured to run at least once every 2 months to avoid the workflow artifact expiring.

/kind misc

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[tekton-robot]

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[AlanGreene]

We'll need fgrosse/go-coverage-report@* added to the actions allow-list so this can run.

[tekton-robot]

The following Tekton test failed:

Test name	Commit	Details	Required	Rerun command
pull-tekton-pipeline-go-coverage-df	940b250	link	true	/test pull-tekton-pipeline-go-coverage-df