fix (install.sh): use realpath instead of script name (#6075)

* fix (install.sh): use realpath instead of script name

###Description:
During arch() sctipt tries to delete itself in case no compatible arch found. This may lead to unexpected file deletion if executed outside root dir; also cur_dir is declared but doesn't seem to be used anywhere

###Way to reproduce:
```bash
cd "/some/other_dir_with_install_sh"
/3x-ui/project/dir/install.sh
```

* fix(install): quote the script path before the self-delete

realpath was handed an unquoted $0, so a script living under a path that
contains spaces was split into several arguments: realpath printed a
partial path plus an error, and rm -f then targeted a name matching
nothing at all. The unsupported-arch branch silently kept the script it
means to remove — the very case the surrounding fix exists for.

install.sh

@@ -6,8 +6,6 @@ blue='\033[0;34m'
 yellow='\033[0;33m'
 plain='\033[0m'
 
-cur_dir=$(pwd)
-
 xui_folder="${XUI_MAIN_FOLDER:=/usr/local/x-ui}"
 xui_service="${XUI_SERVICE:=/etc/systemd/system}"
 
@@ -36,7 +34,7 @@ arch() {
         armv6* | armv6) echo 'armv6' ;;
         armv5* | armv5) echo 'armv5' ;;
         s390x) echo 's390x' ;;
-        *) echo -e "${green}Unsupported CPU architecture! ${plain}" && rm -f install.sh && exit 1 ;;
+        *) echo -e "${green}Unsupported CPU architecture! ${plain}" && rm -f "$(realpath "$0")" && exit 1 ;;
     esac
 }