SG-36291 Do not downgrade startup versions

[julien-lang]

The desktop-startup self-update itself. It downloads the new version from app store, installs it and restarts itself with the new version.

When communicating with the app_store, it will select the latest available version. Even if the version is older than itself.
So in the rare case where the local startup version was up-to-date yesterday but was marked "bad" on app store, startup will downgrade to the previous version.

At the same time, the desktop process will always select the latest startup between the bundled version and the downloaded version. So we get into a restart loop because these 2 algorithms don't have an aligned behaviour.

This PR aligns the behavior but also creates an environment variable to allow older startup version to be downloaded and used.

[carlos-villavicencio-adsk]

I like this. Thanks!
Please add some description to the PR.

[carlos-villavicencio-adsk]

I haven't tested it but can it be rewritten like this?

Suggested change

	if current_version <= app_store_version:
	pass
	elif os.environ.get("SGTK_STARTUP_ALLOW_OLDER_VERSION"):
	pass
	else:
	logger.warning(
	"Ignore app_store version %s since current version is newer %s",
	latest_descriptor.version,
	current_desc.get_version(),
	)
	return False
	if os.environ.get("SGTK_STARTUP_ALLOW_OLDER_VERSION"):
	pass
	elif current_version > app_store_version:
	logger.warning(
	"Ignore app_store version %s since current version is newer %s",
	latest_descriptor.version,
	current_desc.get_version(),
	)
	return False

[julien-lang]

Yes, I think both are equivalent. I wrote it this way because that would be easy to add logs later if needed.
Also I find it easier to read but it's a personal preference.