fix(bots): drop dead PR_BASE_SHA env from engineer follow-up

Review follow-up (F1): reviewer flagged that engineer-bot-followup passes
PR_BASE_SHA without the base-fetch step the reviewer follow-up has. Verified
against the engine: the engineer follow-up (followup_runner.py / run.py) never
reads PR_BASE_SHA — unlike the reviewer, it does no `git rev-list base..head`
anchor verification. So it was dead env, not a missing-base risk. Remove it
rather than adding an unused base-fetch step.

Co-authored-by: Isaac
Signed-off-by: eric-wang-1990 <e.wang@databricks.com>

Author: eric-wang-1990

.github/workflows/engineer-bot-followup.yml

@@ -117,7 +117,6 @@ jobs:
           PR_NUMBER: ${{ github.event.pull_request.number }}
           HEAD_SHA: ${{ github.event.pull_request.head.sha }}
           HEAD_BRANCH: ${{ github.event.pull_request.head.ref }}
-          PR_BASE_SHA: ${{ github.event.pull_request.base.sha }}
           PR_TITLE: ${{ github.event.pull_request.title }}
           PR_URL: ${{ github.event.pull_request.html_url }}
           TRIGGER_COMMENT_ID: ${{ github.event.comment.id }}