buildx's pnpm version doesn't match other actions

When pull-request.yml contains

      - name: Setup pnpm
        uses: pnpm/action-setup@v3
        with:
          version: 9

The build and push action still installed an unpinned pnpm version. Not actually sure what it is since only npm is logged, but the ci had different behavior than local as a result (CI did the supply chain policy check when local didn't)

[44](https://github.com/satisfactorymodding/Documentation/actions/runs/26487807585/job/77998977322#step:6:250)

#13 [build 3/8] RUN npm install -g pnpm
#13 1.313 
#13 1.313 added 1 package in 1s
#13 1.313 
#13 1.314 1 package is looking for funding
#13 1.314   run `npm fund` for details
#13 1.316 npm notice
#13 1.316 npm notice New minor version of npm available! 11.13.0 -> 11.15.0
#13 1.316 npm notice Changelog: https://github.com/npm/cli/releases/tag/v11.15.0
#13 1.316 npm notice To update run: npm install -g npm@11.15.0
#13 1.316 npm notice
#13 DONE 1.4s

#16 [build 6/8] RUN pnpm install
#16 0.907 ? Verifying lockfile against supply-chain policies (499 entries)...
#16 2.594 ✗ Lockfile failed supply-chain policy check (499 entries in 1.6s)

Start with pull-request.yml and trace the build and push action that runs npm install -g pnpm. Compare its pnpm installation with the pinned pnpm/action-setup@v3 step, then verify the build uses the same pnpm version and no longer shows the inconsistent supply-chain policy behavior.