Use whoami instead of $USER (#1843)

Use `whoami` to populate the `user` build info property. 

Docker does not create the envrironment variable USER per
https://docs.docker.com/engine/reference/run/#environment-variables

Signed-off-by: Gera Shegalov <gera@apache.org>

build/build-info

@@ -29,7 +29,7 @@ echo_build_properties() {
   libcudf_path=$3
   shift 3
   echo version=$version
-  echo user=$USER
+  echo user=$(whoami)
   echo revision=$(cd "$git_path" && git rev-parse HEAD)
   echo branch=$(cd "$git_path" && git rev-parse --abbrev-ref HEAD)
   echo date=$(date -u +%Y-%m-%dT%H:%M:%SZ)