feat: graceful fallback for apt-get update failures

Add apt_update_safe() function that warns instead of aborting when apt-get update fails (e.g. enterprise repo 401 Unauthorized). Shows a helpful hint about disabling the enterprise repo when no subscription is active. Replaces direct  apt-get update calls in build.func and install.func.

misc/install.func

@@ -233,7 +233,7 @@ fi
 EOF
     chmod +x /usr/local/bin/apt-proxy-detect.sh
   fi
-  $STD apt-get update
+  apt_update_safe
   $STD apt-get -o Dpkg::Options::="--force-confold" -y dist-upgrade
   rm -rf /usr/lib/python3.*/EXTERNALLY-MANAGED
   msg_ok "Updated Container OS"