Always rebuilding assets with @ in name

[Erich Schubert]

I have javascript assets with an @ in their name, e.g., https://www.npmjs.com/package/@fontsource/fira-sans.

Lektor appears to "rebuild" (i.e., copies) these assets every time, making it unnecessary slow.

(But its not the only thing that makes it slow for large projects, I already have build times of 26+ seconds, unfortunately., c.f. #180 )