Do not call sys.executable in ImageShow in PyInstaller application

[radarhere]

Resolves #9027

The issue has reported that a PyInstaller application does not have a Python instance as sys.executable - it has the application.

https://pyinstaller.org/en/stable/runtime-information.html

When a normal Python script runs, sys.executable is the path to the program that was executed, namely, the Python interpreter. In a frozen app, sys.executable is also the path to the program that was executed, but that is not Python

This means that

Pillow/src/PIL/ImageShow.py

Lines 178 to 185 in ef0bab0

	executable = sys.executable or shutil.which("python3")
	if executable:
	subprocess.Popen(
	[
	executable,
	"-c",
	"import os, sys, time; time.sleep(20); os.remove(sys.argv[1])",
	path,

will not be able to delete the image, and might inadvertently launch the application a second time.

Using a check from https://pyinstaller.org/en/stable/runtime-information.html, this PR ignores sys.executable if in a PyInstaller application. python3 might still be used.