Several run_once scripts unconditionally called sudo pacman/apt to
install packages — even on boxes where every package was already
present. That triggered a sudo password prompt on every fresh
chezmoi apply for nothing.
Two changes:
1. .chezmoi.yaml.tmpl: fall back to ~/.local/bin/age if /usr/bin/age
isn't installed (matters during initial bootstrap before age is
installed system-wide).
2. run_once_*.sh.tmpl: detect missing packages first; only call sudo
if there's actually something to install. For the LAN hosts script,
detect the existing block and skip if it's already correct.
These changes are transparent on boxes that already had everything
installed (the existing 5): no behavior change. They reduce sudo
prompts on bit (the new box, where most packages are pre-installed)
from ~5 prompts to 1 (just for /etc/hosts).
