2.0.0rc8: useless npm install & npm run build if built from sdist

[frenzymadness]

When I run a build on top of the sources downloaded from PyPI (sdist) the build process depends on npm and runs npm install and npm run build which downloads 41 nodejs modules from internet. I believe this is unnecessary because it literally changes nothing in the source tree as the minimal CSS files are already present in the sdist.

Would it be possible to skip this step if the artifacts it should create already exist?

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