chroot: replace devtools functionality

[AladW]

Without having to rely on devtools, arch-install-scripts and its various quirks, aur-chroot could be extended in multiple ways. For example:

• Handle CacheDir reliably (aur-chroot: writable bind mount for local repo cache, fixed with devtools patch #809)
• Move the complete build process to the container (GPG keyrings included), instead of retrieving sources on the host
• Support overlayfs (aur-chroot: use overlayfs for temp containers #705)
• Support custom systemd-nspawn arguments (aur-chroot: increase /tmp size in chroot build environment #1032, Trying to build telegraf in nspawn chroot fails due to depedency's requirement of IPC_LOCK capability #1044)
• Ensure external tools like pacserve work (aur chroot: uses pacserve to obtain *.db #699)
• Use pacutils-sysroot instead of pacstrap (bugged arch-install-script break chroot build #1015)
• Avoid trivial privilege escalation vectors (https://gitlab.archlinux.org/archlinux/devtools/-/merge_requests/114)
• Make container makepkg configuration independent from the host (--makepkg-conf=FILE broken for chroot builds #1034)
• Avoid ambiguities between host and container makepkg.conf (build: --chroot --pkgver does not respect SRCDEST and BUILDDIR from makepkg.conf #1102)

For a prototype, see https://github.com/AladW/aurutils/blob/chroot-standalone/lib/aur-chroot