Earlier quoted context omitted.
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.
Cargo is a cross-platform tool, so when it ships a sandboxing solution it will need to be a cross-platform solution, and because this is a security feature it needs to be bulletproof, so no half-measures like Docker. Something like a WASM runtime might fit the bill, though that will be much easier to get working for typical proc macros than for typical build scripts. If you only care about Unix, then you can do this yourself today by building code in your sandbox of choice.