chore(deps): bump pnpm/action-setup from 4 to 6

Bumps [pnpm/action-setup](https://github.com/pnpm/action-setup) from 4 to 6.
- [Release notes](https://github.com/pnpm/action-setup/releases)
- [Commits](pnpm/action-setup@v4...v6)

---
updated-dependencies:
- dependency-name: pnpm/action-setup
  dependency-version: '6'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

.github/workflows/nightly.yml

@@ -31,7 +31,7 @@ jobs:
           node-version: '24'
 
       - name: Setup pnpm
-        uses: pnpm/action-setup@v4
+        uses: pnpm/action-setup@v6
 
       # Scaffold the published basic (SQLite, no-auth) template fully
       # non-interactively into an OS temp dir — never the runner's workspace —