dts output is deleted on clean with multiple config

This causes https://github.com/storybookjs/addon-kit/issues/73

The config there is [an array](https://github.com/storybookjs/addon-kit/blob/main/tsup.config.ts): `export default defineConfig(async () => [])`.

Only the `.d.ts` from the last config is retained. The ones from previous configs are deleted.

At the same time, the `.js` output are correctly retained.

Reproduce the issue using the multiple-config example in tsup.config.ts from the linked addon-kit issue, comparing the generated .d.ts and .js outputs after a clean. Trace the clean step that removes declaration files; done means every .d.ts from all configs is retained, matching the current .js behavior.

Written by the indexing model from the issue text.