It's less NIH and more "cache your dependencies." Details will very greatly depending on what your tech stack looks like, if you're lucky you can just inline a cache. I know Artifactory is a relatively general commercial solution although I can't speak personally about it.
If you can't easily use an existing caching solution, then the only NIH you need to do is copying files that your build system downloads. I know many build systems are "just a bunch of scripts" so those would probably be pretty amenable to this, I don't know if more opaque systems exist that wouldn't give you any access like that. If so, I suppose you could try to just copy the disk the build system writes everything to, but then you're getting into pretty hacky stuff and that's not ideal. Copying the files doesn't give you the nice UX of a cache, but it does mean that in the worst case scenario you at least have the all the dependencies you've used in recent builds, so you'll be able to keep building your things.
> I don't know if more opaque systems exist that wouldn't give you any access like that
As long as there is there is "server reimplementation", i.e. private registries available, one can always hack together a solution out of self signed CA, DNS and routing to replace "the server" with local registry.
If you can't easily use an existing caching solution, then the only NIH you need to do is copying files that your build system downloads. I know many build systems are "just a bunch of scripts" so those would probably be pretty amenable to this, I don't know if more opaque systems exist that wouldn't give you any access like that. If so, I suppose you could try to just copy the disk the build system writes everything to, but then you're getting into pretty hacky stuff and that's not ideal. Copying the files doesn't give you the nice UX of a cache, but it does mean that in the worst case scenario you at least have the all the dependencies you've used in recent builds, so you'll be able to keep building your things.