Inform the user that ue_root is not set

[carryaimp]

Fixes: #83

About

Inform the user that the environment variable ue_root is not set.

How Has This Been Tested?

1. If the ue_root environment variable is not set, does executing the setup_linux_dev_tools.sh and build.sh scripts result in an error message and terminate the scripts?

unset UE_ROOT
bash -x ./setup_linux_dev_tools.sh
bash -x ./build.sh simlibs_debug
# The program will display a message in the standard output and then exit.

2. Set the environment variable Ue_root, execute the setup_linux_dev_tools.sh and build.sh scripts, and the scripts execute successfully.

export UE_ROOT=/home/carryaimp/apps/Linux_Unreal_Engine_5.2.1
bash -x ./setup_linux_dev_tools.sh

bash -x ./build.sh clean
bash -x ./build.sh simlibs_debug

Screenshots and videos (if appropriate):