fix(sdk) orig_params being overwritten in custom-tasks

[Udiknedormin]

Which issue is resolved by this Pull Request:
In some rare cases (difficult to reproduce), pipeline params in custom tasks are not recognised as orig_params and therefore $(inputs.params.foo-bar) is produced instead of $(params.foo-bar).

Description of your changes:
When custom task spec is overwritten from its taskSpec, all fields were pasted as-is. However, one of those was "orig_params": task_ref["params"] --- which works differently the first and the second time it's called, thus dropping the actual information on original parameters.

This PR factors out that variable and pastes it in both places without dropping its previous value.

Environment tested:

• Python Version (use python --version): 3.9.0
• Tekton Version (use tkn version): irrelevant
• Kubernetes Version (use kubectl version): irrelevant
• OS (e.g. from /etc/os-release): irrelevant

Checklist:

• The title for your pull request (PR) should follow our title convention. Learn more about the pull request title convention used in this repository.

[Tomcli]

/lgtm
/approve

[google-oss-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Tomcli, Udiknedormin

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