Phase 6A: remove retained PAT shadows from develop workflows

[ihalatci]

Phase 6A: remove the retained dead secrets.GH_TOKEN top-level env: shadow line from 6 already-migrated workflows (pure deletions, one line per file):

• build-package.yml, build-package-test.yml, image-health-check.yml (quoted shadow removed)
• build-citus-community-nightlies.yml, build-pgazure-nightlies.yml, update-pgxn-version.yml (unquoted shadow removed)

Rationale: each of these workflows now mints its own GitHub App token via actions/create-github-app-token@v3 and exports it to $GITHUB_ENV, so the top-level PAT shadow was unused; the org PAT secrets.GH_TOKEN is retired/dead. diff --stat = 6 files changed, 6 deletions(-), 0 insertions(+); grep secrets.GH_TOKEN across .github/workflows/ = 0 matches.

Validation (push-triggered on 3869af0, all success):

• Build Package — run 28934598464 — success
• Build package for test images — run 28934598467 — success
• Image Health Check — run 28934598520 — success
• Build and publish citus community nightly packages — run 28934598492 — success
• build-pgazure-nightlies (disabled_manually) and update-pgxn-version (workflow_dispatch-only) did not fire — expected.

MERGE HELD — draft for review. No merge, no auto-merge.