This protects your machine but not necessarily the resulting binary code. Nothing prevents the malware from patching the source before building and removing the

This protects your machine but not necessarily the resulting binary code. Nothing prevents the malware from patching the source before building and removing the patch before the end, resulting in a binary and a package that contain code that doesn't match your trusted sources. As a general rule, it must always be possible to build without network access, otherwise you're having a big trust problem. And once you know you're having all your dependencies, only then you can think about sandboxing all this stuff after having carefully studied it.