Secure execution of build scripts

[mtshiba]

The erg script specified in poise's pre-build can currently execute arbitrary code. Although this is unavoidable to some extent, it does pose a security risk.

https://blog.rust-lang.org/inside-rust/2023/09/01/crates-io-malware-postmortem.html

The following are some of the counterplans I can think of now:

• Create a sandbox environment and only allow execution within it
• Analyze the effects of build scripts to make it clear to the user whether the script is creating files or communicating externally, etc.