install.sh ran as root and created /tmp/aic8800d80_install.log with mode
666 at a fixed, predictable path. Root writing to a fixed name in /tmp is
open to symlink races (mitigated, not removed, by fs.protected_symlinks),
and a world-writable log lets any local user alter what an administrator
later reads.
The log now lives at /var/log/aic8800-install.log, is created with mode
0644 only after the root check, and failed writes before that point are
silenced instead of printing redirection errors. diagnostic_build.sh
reads the new path and falls back to the old one for logs left by
earlier installers.
(cherry picked from commit ef66811ca5)
