Avoid :eacces by running ESP-IDF esptool.py via python

[mnishiguchi]

Description

Fixes a :eacces error when running mix atomvm.esp32.flash in an environment where IDF_PATH is set.

Environment

• OS: Linux (fish)
• Elixir: 1.17.3 (Erlang/OTP 27)
• ESP-IDF: 5.4 (IDF_PATH=/home/mnishiguchi/esp/esp-idf)
• Python: ESP-IDF venv (python3, python)
• ESP-IDF esptool.py is not executable:

$ ls -l $IDF_PATH/components/esptool_py/esptool/esptool.py
.rw-rw-r-- ... /home/mnishiguchi/esp/esp-idf/components/esptool_py/esptool/esptool.py

Error observed

Flashing using esptool..
** (ErlangError) Erlang error: :eacces
    :erlang.open_port(
      {:spawn_executable,
       ".../esp-idf/components/esptool_py/esptool/esptool.py"},
      [...]
    )

Cause

When IDF_PATH is set, ExAtomVM uses ESP-IDF’s bundled esptool.py. In some setups (including the ESP-IDF 5.4 environment above), this script is not executable, so attempting to execute it directly fails with :eacces.

Fix

When IDF_PATH is set, execute ESP-IDF’s esptool.py via python3/python instead of executing the script directly. The task now also fails fast with concise, actionable errors when esptool.py or python is missing.

Test

• IDF_PATH unset → uses esptool.py from PATH
• IDF_PATH set → runs python <esptool.py> ... without :eacces

Notes

• Using the Pythonx-based flow avoids this, but since pythonx is optional, users on the system esptool path—especially with IDF_PATH set—can still hit :eacces. This PR hardens that path.