refactor(tools): shellcheck pass modification

Closes https://github.com/espressif/esp-idf/pull/12247

export.sh

@@ -19,7 +19,7 @@ __verbose() {
 
 __script_dir(){
     # shellcheck disable=SC2169,SC2169,SC2039,SC3010,SC3028  # unreachable with 'dash'
-    if [ `uname -s` = "Darwin" ]; then
+    if [ "$(uname -s)" = "Darwin" ]; then
         # convert possibly relative path to absolute
         script_dir="$(__realpath "${self_path}")"
         # resolve any ../ references to make the path shorter