Make update check faster

[passsy]

sidekick checks for new sidekick updates after a command is executed.

sidekick/sidekick_core/lib/sidekick_core.dart

Lines 162 to 164 in 914bde5

	// TODO start the update check in the background at command start
	// TODO prevent multiple update checks when a command start another command
	await _checkForUpdates();

It would be faster to check for updates while the command is executing and print the result in the end.