feature: add a python-path output

Expose a `python-path` output containing the chosen Python executable path.

__tests__/check-python-path.sh

@@ -0,0 +1,14 @@
+#!/bin/bash
+
+set -euo pipefail
+
+PYTHON_PATH="$1"
+PATH_EXECUTABLE=$(python -c 'import sys; print(sys.executable)')
+PYTHON_PATH_EXECUTABLE=$("${PYTHON_PATH}" -c 'import sys; print(sys.executable)')
+if [ "${PATH_EXECUTABLE}" != "${PYTHON_PATH_EXECUTABLE}" ]; then
+    echo "Executable mismatch."
+    echo "python in PATH is: ${PATH_EXECUTABLE}"
+    echo "python-path (${PYTHON_PATH}) is: ${PYTHON_PATH_EXECUTABLE}"
+    exit 1
+fi
+echo "python-path: ${PYTHON_PATH}"