Improve error handling and logging for LXC builds (#12208)

Prevent host-side error_handler from being triggered during in-container install/recovery by delaying re-enabling set -Eeuo pipefail and the ERR trap in misc/build.func until after install/recovery completes; add explanatory comments. Update misc/error_handler.func to fall back to BUILD_LOG if container-internal log path is unavailable, show the last 20 log lines when present, refine container vs host detection (check INSTALL_LOG file and /root), copy INSTALL_LOG into /root and write a .failed flag with the exit code for host-side detection, and ensure full-log output and container removal prompt are shown appropriately in host context. Tweak misc/core.func silent() output to include a "Full log" path and adjust formatting.

misc/core.func

@@ -524,7 +524,8 @@ silent() {
     if [[ -s "$logfile" ]]; then
       echo -e "\n${TAB}--- Last 10 lines of log ---"
       tail -n 10 "$logfile"
-      echo -e "${TAB}-----------------------------------\n"
+      echo -e "${TAB}-----------------------------------"
+      echo -e "${TAB}📋 Full log: ${logfile}\n"
     fi
 
     exit "$rc"