Sandboxing for build scripts can't work properly. If you sandbox too much, some necessary stuff can't be done. If you sandbox too little, it has no practical value.

As an easy start, how about letting build scripts read /usr, read and write a temporary build directory, have some /tmp scratch space, and be allowed to write its final output artifact. No network and otherwise isolated from the rest of the system.

I would argue that, if a build script doesn’t work in the setting, then it doesn’t deserve to be installable by a default cargo command.

So let each build script define its own level of sandboxing and then users can determine whether they are okay with that level or not, e.g. `cargo build --sandbox-level=...`