I don't mind code execution as even make has shell scripting embedded, same thing with meson. My issue is with network access to download random stuff without it being declared somewhere and signed.
I'm using OpenBSD and the ports system set up two users `_pbuild` and `_pfetch` for building packages. The first one is for building and the sample `pf.conf` (firewall) forbid it from accessing the network. The second does fetch the files , but it's declarative with every files listed and signed. Even for languages like go and rust.
Fetching files while building is the bad idea there.
Good point. Many build systems also want to be a package manager, but the two are and should be separate. If you have a simple problem it makes things easy to combine them into one. However they need to be separate anyway, both for security and also to make other weird situations easier.