chore(ci): remove dead env vars from circleci-failure-summary-comment.yml (#45972)

`TARGET_BRANCH` and `TARGET_SHA` were declared at the job level but never
referenced by any subsequent step. `PR_NUMBER` was also declared at the
job level but every step that needs it re-declares its own step-local
`PR_NUMBER: ${{ github.event.pull_request.number }}` env, so the
job-level binding was equally unused.

Drop the entire job-level `env:` block. No behavior change.

Author: XciD

.github/workflows/circleci-failure-summary-comment.yml

@@ -12,10 +12,6 @@ jobs:
     runs-on: ubuntu-22.04
     permissions:
       pull-requests: write
-    env:
-      TARGET_BRANCH: ${{ github.event.pull_request.head.ref }}
-      TARGET_SHA: ${{ github.event.pull_request.head.sha }}
-      PR_NUMBER: ${{ github.event.pull_request.number }}
     steps:
       - name: Checkout repository
         uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd  # v6.0.2