Write the installer log to /var/log instead of a world-writable /tmp file

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)
(cherry picked from commit 33d33afe96)

diagnostic_build.sh

@@ -19,7 +19,11 @@ NC='\033[0m'
 
 DRV_NAME="aic8800"
 DRV_VERSION="1.0.0"
-INSTALL_LOG="/tmp/aic8800d80_install.log"
+INSTALL_LOG="/var/log/aic8800-install.log"
+# Installers before this change wrote their log to /tmp.
+if [ ! -f "$INSTALL_LOG" ] && [ -f /tmp/aic8800d80_install.log ]; then
+    INSTALL_LOG="/tmp/aic8800d80_install.log"
+fi
 
 read_dkms_conf_value() {
     local key="$1"