It isn't so much a question of sandboxing build.rs, as fundamentally changing the way that foreign dependencies are integrated into the rust toolchain (i.e. moving from a rust-centric system like Cargo to something more general like buck2)
To solve the problem of native/C dependencies, they just bundle pre-compiled libraries as data files.
Rust could do the same thing.
It doesn't really matter anyway, because nobody is reading anything in their dependencies before it gets downloaded. Malware can also be hiding in plain sight in source code, as this attack and many others shows.
Which is the result of how bad dependency managment is outside (i.e. DLL-hell).
Pretty much all other use cases of build.rs can be sandboxed. Well, there is sqlx that wants to connect to database at expansion time to compile check-queries (yew).
Although I suppose that's still doing a lot of shenanigans at compile time. It could be sandboxed pretty well (theoretically). I'd hate to give it up completely though, getting a compile time error when SELECT query params or return values have type mismatches is extremely nice.
Any C library that's also packaged by debian supports being built under these conditions because it's required for everything except non-free packages: https://www.debian.org/doc/debian-policy/ch-source.html#main...
As the link you posted mentions, you need a tiny bit more than that: you also need write access to the temporary directory (/tmp and similar). Many build tools temporarily store files there; for instance, unless things have changed since I last looked, if you don't use the -pipe argument the C compiler stores its temporary intermediate files (preprocessor output, assembler input) there.