[nix] bump mill to 0.12.5 #1446

Job	Run time
Generate test plan	4s
Build VCS Emulators (blastoise)	14s
Build VCS Emulators (rookidee)	12s
Prepare for running testcases	4s
Run VCS (1)	8s
Run VCS (2)	6s
Run VCS (3)	8s
Run VCS (4)	7s
Run VCS (5)	8s
Run VCS (6)	6s
Run VCS (7)	6s
Run VCS (8)	8s
Run VCS (9)	8s
Run VCS (10)	7s
Run VCS (11)	7s
Run VCS (12)	7s
Run VCS (13)	6s
Run VCS (14)	7s
Run VCS (15)	7s
Run VCS (16)	7s
Run VCS (17)	7s
Run VCS (18)	7s
Run VCS (19)	7s
Run VCS (20)	6s
Run VCS (21)	7s
Run VCS (22)	7s
Run VCS (23)	8s
Run VCS (24)	7s
Run VCS (25)	6s
Run VCS (26)	6s
Run VCS (27)	7s
Run VCS (28)	6s
Run VCS (29)	6s
Run VCS (30)	7s
Run VCS (31)	7s
Run VCS (32)	8s
Run VCS (33)	7s
Run VCS (34)	6s
Run VCS (35)	7s
Run VCS (36)	7s
Report VCS CI result	16s
	4m 59s

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