ci: force bun runtime for tsdown on bun jobs

`bunx tsdown` honors tsdown's `#!/usr/bin/env node` shebang and runs the
binary under node, so tsdown's loader auto-detection falls back to `unrun`
(an optional peer dep that isn't installed). `--bun` overrides the shebang
and runs tsdown under bun, where the native TS loader works.

Co-Authored-By: Claude (Opus 4.7) <noreply@anthropic.com>

Author: silverwind

.github/workflows/ci.yaml

@@ -31,5 +31,5 @@ jobs:
         if: ${{matrix.js[0] == 'bun'}}
       - run: make deps lint test
         if: ${{matrix.js[0] == 'node'}}
-      - run: bun install && bunx tsdown && bun test --timeout 180000
+      - run: bun install && bunx --bun tsdown && bun test --timeout 180000
         if: ${{matrix.js[0] == 'bun'}}