Tools: Use default value in the Python detections scripts

Closes https://github.com/espressif/esp-idf/issues/8549

tools/detect_python.fish

@@ -5,6 +5,8 @@
 set OLDEST_PYTHON_SUPPORTED_MAJOR 3
 set OLDEST_PYTHON_SUPPORTED_MINOR 7
 
+set -x ESP_PYTHON python
+
 for p_cmd in python3 python python3.7 python3.8 python3.9 python3.10 python3.11 python3.12;
     $p_cmd --version >/dev/null 2>&1; or continue
     echo "Checking \"$p_cmd\" ..."

tools/detect_python.sh

@@ -10,6 +10,8 @@
 OLDEST_PYTHON_SUPPORTED_MAJOR=3
 OLDEST_PYTHON_SUPPORTED_MINOR=7
 
+ESP_PYTHON=python
+
 for p_cmd in python3 python python3.7 python3.8 python3.9 python3.10 python3.11 python3.12; do
     $p_cmd --version >/dev/null 2>&1 || continue
     echo "Checking \"$p_cmd\" ..."