[build] docker-sonic-mgmt replace USER by whoami (#9702)

Author: liushilongbuaa

src/sonic-build-hooks/scripts/buildinfo_base.sh

@@ -17,7 +17,7 @@ REPR_MIRROR_URL_PATTERN='http:\/\/packages.trafficmanager.net\/debian'
 
 URL_PREFIX=$(echo "${PACKAGE_URL_PREFIX}" | sed -E "s#(//[^/]*/).*#\1#")
 
-if [ $USER != 'root' ] && [ -n $(which sudo) ];then
+if [ "$(whoami)" != "root" ] && [ -n "$(which sudo)" ];then
     SUDO=sudo
 else
     SUDO=''