[nix] bump mill to 0.12.5 #1429

Job	Run time
Generate test plan	2m 14s
Build VCS Emulators (blastoise)	27s
Build VCS Emulators (rookidee)	40s
Prepare for running testcases	5s
Run VCS (1)	3m 52s
Run VCS (2)	1m 58s
Run VCS (3)	21s
Run VCS (4)	26s
Run VCS (5)	27s
Run VCS (6)	38s
Run VCS (7)	1m 8s
Run VCS (8)	2m 23s
Run VCS (9)	24s
Run VCS (10)	1m 25s
Run VCS (11)	19s
Run VCS (12)	14s
Run VCS (13)	1m 19s
Run VCS (14)	17s
Run VCS (15)	1m 46s
Run VCS (16)	18s
Run VCS (17)	1m 5s
Run VCS (18)	2m 8s
Run VCS (19)	1m 42s
Run VCS (20)	2m 7s
Run VCS (21)	1m 45s
Run VCS (22)	1m 7s
Run VCS (23)	1m 57s
Run VCS (24)	2m 11s
Run VCS (25)	1m 1s
Run VCS (26)	2m 25s
Run VCS (27)	1m 33s
Run VCS (28)	37s
Run VCS (29)	2m 17s
Run VCS (30)	1m 42s
Run VCS (31)	1m 18s
Run VCS (32)	2m 13s
Run VCS (33)	1m 7s
Run VCS (34)	1m 30s
Run VCS (35)	1m 1s
Run VCS (36)	1m 55s
Report VCS CI result	35s
	53m 57s

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