Measure install time with monotonic clock instead of system clock

Using time.time() can be inaccurate if the system clock gets
updated in between calls.

Author: correctmost

archinstall/scripts/guided.py

@@ -57,7 +57,7 @@ def perform_installation(
 	Only requirement is that the block devices are
 	formatted and setup prior to entering this function.
 	"""
-	start_time = time.time()
+	start_time = time.monotonic()
 	info('Starting installation...')
 
 	config = arch_config_handler.config
@@ -169,7 +169,7 @@ def perform_installation(
 		debug(f'Disk states after installing:\n{disk_layouts()}')
 
 		if not arch_config_handler.args.silent:
-			elapsed_time = time.time() - start_time
+			elapsed_time = time.monotonic() - start_time
 			action = select_post_installation(elapsed_time)
 
 			match action: