There's a deeper issue though. I frequently have difficult getting things to build from source in a network isolated environment. That's after I manually wrangle all the dependencies (and sub-deps, and sub-sub-deps, and ...).
Even worse is something like emscripten where you are fully expected to run `npm install`.
Any build process that depends on network access is fundamentally broken as far as I'm concerned.
You can cache and/or emulate the network to go offline but fundamentally a fresh build in most languages will want to hit a network at least by default
The real problem is that some language ecosystems conflate those two steps.
Even in C/C++ after changing the relevant parameters to non-default values things often break. It seems those configurations often go untested.
Google managed repos are a nice exception to this. Clearly documented commit hashes for all dependencies for a given release.