fix: add bun setup to nightly workflow for vscode-ext build

The nightly job runs `make release` which depends on `vscode-ext`,
requiring bun. Added `oven-sh/setup-bun@v2` step matching the
release workflow.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: ako

.github/workflows/nightly.yml

@@ -49,6 +49,8 @@ jobs:
         with:
           go-version: '1.26'
 
+      - uses: oven-sh/setup-bun@v2
+
       - name: Check for new commits since last nightly
         id: check
         run: |