Feature: Use npm cache add instead of npm install in fetch

[Kixunil]

Is your feature request related to a problem? Please describe.
Running npm install before building has several disadvantages:

• Takes a long time which makes build-dep step more annoying
• Makes it impossible to properly isolate builds
• Can not be configured per-package
• May be causing rebuild issues
• It's an ugly exception in the architecture

Describe the solution you'd like
What we wanted to achieve - downloading dependencies - should be achievable using npm cache add. We can build it later with npm install

Describe alternatives you've considered

• We could get rid of vendoring completely but that would mean perfect build isolation is impossible
• We could hand-vendor the dependencies (e.g. as Debian packages) - very time-consuming even though also very clean.

Additional context
Required if #82 is revisited