tools: Prefer python3 during install and export

Install and export script should work on systems without "python"
executable.

Closes https://github.com/espressif/esp-idf/pull/6471

Closes https://github.com/espressif/esp-idf/issues/6532

Related to https://github.com/espressif/esp-idf/issues/6421 and
https://github.com/espressif/arduino-esp32/issues/4717

export.fish

@@ -8,11 +8,14 @@ function idf_export_main
 
     set oldpath = $PATH
 
+    echo "Detecting the Python interpreter"
+    source "$IDF_PATH"/tools/detect_python.fish
+
     echo "Adding ESP-IDF tools to PATH..."
     # Call idf_tools.py to export tool paths
     set -x IDF_TOOLS_EXPORT_CMD "$IDF_PATH"/export.fish
     set -x IDF_TOOLS_INSTALL_CMD "$IDF_PATH"/install.fish
-    set idf_exports ("$IDF_PATH"/tools/idf_tools.py export) || return 1
+    set idf_exports ("$ESP_PYTHON" "$IDF_PATH"/tools/idf_tools.py export) || return 1
     eval "$idf_exports"
 
     echo "Checking if Python packages are up to date..."
@@ -50,6 +53,7 @@ function idf_export_main
     set -e path_entry
     set -e IDF_ADD_PATHS_EXTRAS
     set -e idf_exports
+    set -e ESP_PYTHON
 
     # Not unsetting IDF_PYTHON_ENV_PATH, it can be used by IDF build system
     # to check whether we are using a private Python environment