Rewrite bootstrap creation script

- Changed it to use namespaces to allow it to run without root privileges on systems that have user namespaces enabled (see also minor fixes that done along with that commit in commit message) - Rewritten it to fully separate parts that run inside chroot from parts that run outside of it. This is mainly due to not being able to persist namespace mount between commands without root privileges but also that I think it is cleaner that way. - Allowed disabling Chaotic-AUR repository. - Moved all build files to /build sub directory. - Made it so that pacman host cache is mounted in chroot when it is available to speed up chroot build process.