build_image_env() should not do pacman -Syy on the host system

[digitalsignalperson]

I'm keen to try junest and was just doing the following to get started:

• clone repo
• add junest/bin to path
• run junest build since I prefer to not download the archive blob from somewhere

noticed immediately being asked for sudo to do sudo pacman --noconfirm -Syy

but isn't this a partial upgrade and why would it be necessary to do on my host system? As far as I can tell this is not in a container/namespace yet. I'd rather do sudo pacman -Syu on my own beforehand if necessary and not let junest potentially break my system with -Syy. Let me know if I'm misunderstanding this.

[digitalsignalperson]

I should have just followed https://github.com/fsquillace/junest/wiki/How-to-build-a-JuNest-image-using-QEMU