chore(FN-4105): remove Bun setup from sharded CI jobs

- Remove the Bun install step from the deterministic test shard job in .github/workflows/ci.yml
- Remove the matching Bun install step from the PR checks shard job in .github/workflows/pr-checks.yml
- Keep shard test execution on the shared setup-node-pnpm action only

Fusion-Task-Id: FN-4105

.github/workflows/pr-checks.yml

@@ -69,8 +69,5 @@ jobs:
       - name: Setup Node.js and pnpm
         uses: ./.github/actions/setup-node-pnpm
 
-      - name: Install Bun
-        uses: oven-sh/setup-bun@v2
-
       - name: Test (deterministic shard)
         run: pnpm test:ci:shard --shard ${{ matrix.shard }} --total 3