HDFS-17912. HttpFS: honor HTTPFS_CONFIG when sourcing httpfs-env.sh

hadoop-httpfs.sh sourced httpfs-env.sh from HADOOP_CONF_DIR only,
even though HTTPFS_CONFIG is already honored for -Dhttpfs.config.dir
in the same script. Use ${HTTPFS_CONFIG:-${HADOOP_CONF_DIR}} so both
paths agree. Backward compatible.

Author: eubnara

hadoop-hdfs-project/hadoop-hdfs-httpfs/src/main/libexec/shellprofile.d/hadoop-httpfs.sh

@@ -25,9 +25,9 @@ fi
 ## @replaceable  no
 function hdfs_subcommand_httpfs
 {
-  if [[ -f "${HADOOP_CONF_DIR}/httpfs-env.sh" ]]; then
+  if [[ -f "${HTTPFS_CONFIG:-${HADOOP_CONF_DIR}}/httpfs-env.sh" ]]; then
     # shellcheck disable=SC1090
-    . "${HADOOP_CONF_DIR}/httpfs-env.sh"
+    . "${HTTPFS_CONFIG:-${HADOOP_CONF_DIR}}/httpfs-env.sh"
   fi
 
   # shellcheck disable=SC2034