ci: replace secrets: inherit with explicit secrets in chatops_retest

[vdemeester]

Problem

zizmor reports secrets-inherit in chatops_retest.yaml. The workflow uses secrets: inherit which passes all parent secrets to the reusable workflow tektoncd/plumbing/.github/workflows/_chatops_retest.yml, when it only needs CHATOPS_TOKEN.

Current

# .github/workflows/chatops_retest.yaml
jobs:
  retest:
    uses: tektoncd/plumbing/.github/workflows/_chatops_retest.yml@48c53b4e...  # main
    secrets: inherit

Proposed fix

jobs:
  retest:
    uses: tektoncd/plumbing/.github/workflows/_chatops_retest.yml@48c53b4e...  # main
    secrets:
      CHATOPS_TOKEN: ${{ secrets.CHATOPS_TOKEN }}

Note: This also requires a change in tektoncd/plumbing to declare secrets: inputs in _chatops_retest.yml:

on:
  workflow_call:
    secrets:
      CHATOPS_TOKEN:
        required: true

Context

Identified by zizmor v1.23.1. Related to #9667.

/kind cleanup