svm-rs cannot be built offline

[farcaller]

svm-rs cannot be built offline because the build.rs requires internet connection to fetch the json with the versions. It makes it impossible to e.g. package anything that depends on svm-rs in nix, as the builder won't allow the outgoing http connections.

Please provide an alternative mechanism for allowing to pass the list offline. Currently, there's the _offline feature, but it will just result in an empty list. I suggest there's an environment variable that points to a pre-downloaded list (that's potentially checked into the version control elsewhere), and svm-builds should use it first (if the variable is set), falling back to fetching it online otherwise.

[mattsse]

agree this would be ideal
We just haven't gotten around to add this. I guess an easy solution would be to write a test that generates all files and the build script uses that as fallback

Do you want to take this on?