Does't coped fonts in build --watch mode

[cawa-93]

Describe the bug

In the build --watch mode, font files are added to assets only during the first build. When rebuilding after changes, font files are deleted and not added back.

Reproduction

1. Clone this repo: https://github.com/cawa-93/vite-assets-issue/
2. Run npm ci
3. Run npm run watch
4. Check dist folder after first build. It should contain font files.
5. Make any changes in styles.css to trigger rebuild. Check dist folder agine.

System Info

Output of npx envinfo --system --npmPackages vite,@vitejs/plugin-vue --binaries --browsers:

  System:
    OS: Windows 10 10.0.19042
    CPU: (16) x64 AMD Ryzen 7 4800H with Radeon Graphics
    Memory: 4.68 GB / 15.42 GB
  Binaries:
    Node: 14.16.1 - C:\Program Files\nodejs\node.EXE
    Yarn: 1.22.10 - ~\AppData\Roaming\npm\yarn.CMD
    npm: 7.10.0 - C:\Program Files\nodejs\npm.CMD
  Browsers:
    Edge: Spartan (44.19041.423.0), Chromium (90.0.818.51)
    Internet Explorer: 11.0.19041.1
  npmPackages:
    vite: ^2.2.4 => 2.2.4

Used package manager: npm

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[cawa-93]

This issue should be resolved by #3145