fix(install): use runuser and fail non-root with exit 1

`su -` runs through pam_systemd on Manjaro/Arch, creating a new login
session that conflicts with the outer sudo session and detaches the
install from the controlling terminal — making `sudo make install`
appear to end while it keeps running in the background. Replace `su`
calls with `runuser`, which is designed for root-invoked scripts and
skips PAM session management.

Also flips init.sh's non-root branch from `exit 0` (silent success) to
`exit 1` with a clear stderr message, so `make install` correctly fails
when invoked without root.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

scripts/installation/arch/aur-builder-setup.sh

@@ -38,7 +38,7 @@ echo "[aur-builder-setup] Configuring sudoers for aur_builder..."
 ${ROOT_CMD} bash -c "echo '%aur_builder ALL=(ALL) NOPASSWD: /usr/bin/pacman' > /etc/sudoers.d/aur_builder"
 ${ROOT_CMD} chmod 0440 /etc/sudoers.d/aur_builder
 
-RUN_AS_AUR=(su - aur_builder -s /bin/bash -c)
+RUN_AS_AUR=(runuser -u aur_builder -- bash -c)
 
 echo "[aur-builder-setup] Ensuring yay is installed for aur_builder..."