Replace deprecated egrep with grep -E

[mudern]

Summary

• Replaced deprecated egrep with grep -E in shell scripts
• Updated EGREP_PATH to point to grep and added the -E flag where the extended-regex alternation (pkgsets|gos|bin) is used
• Updated the corresponding error message to reference grep -E

Why

egrep is deprecated by POSIX. grep -E is the modern equivalent and is more portable across platforms (e.g. some ARM / minimal environments may not ship a standalone egrep binary).