chore(ci): pin lint-style-action to a SHA instead of @main (#561)

This PR pins `leanprover-community/lint-style-action` to commit
`91923b04` (2026-05-12) instead of tracking `@main`. SHA-pinning
third-party GitHub Actions is the standard hardening practice — it
prevents a compromised or accidentally-broken upstream commit from
silently affecting CI here, and makes the action version reproducible.

The chosen pin matches what mathlib4 uses (after the companion bump in
leanprover-community/mathlib4#39228) and
includes
leanprover-community/lint-style-action#5 -
feat(install-bibtool): use --no-install-recommends with apt-get, which
trims ~167 MB of unused texlive packages off every run of the Lint style
step.

Going forward, bumping the pin can be a periodic, reviewed step rather
than an implicit dependency on whatever HEAD happens to be.

🤖 Prepared with Claude Code

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: kim-em

.github/workflows/lean_action_ci.yml

@@ -34,6 +34,6 @@ jobs:
         run: |
           set -e
           lake exe checkInitImports
-      - uses: leanprover-community/lint-style-action@main
+      - uses: leanprover-community/lint-style-action@91923b046cc6a468e9ae340f32f8defe586bd08f # 2026-05-12
         with:
           mode: check