[nix] bump mill to 0.12.5 #1441

Job	Run time
Generate test plan	7s
Build VCS Emulators (blastoise)	23s
Build VCS Emulators (rookidee)	46s
Prepare for running testcases	39s
Run VCS (1)	2m 23s
Run VCS (2)	37s
Run VCS (3)	1m 45s
Run VCS (4)	35s
Run VCS (5)	1m 50s
Run VCS (6)	1m 53s
Run VCS (7)	2m 1s
Run VCS (8)	2m 57s
Run VCS (9)	1m 38s
Run VCS (10)	27s
Run VCS (11)	1m 41s
Run VCS (12)	28s
Run VCS (13)	2m 22s
Run VCS (14)	32s
Run VCS (15)	41s
Run VCS (16)	1m 35s
Run VCS (17)	1m 43s
Run VCS (18)	23s
Run VCS (19)	19s
Run VCS (20)	32s
Run VCS (21)	42s
Run VCS (22)	1m 25s
Run VCS (23)	1m 46s
Run VCS (24)	1m 56s
Run VCS (25)	1m 27s
Run VCS (26)	30s
Run VCS (27)	2m 4s
Run VCS (28)	1m 42s
Run VCS (29)	2m 25s
Run VCS (30)	24s
Run VCS (31)	1m 42s
Run VCS (32)	1m 6s
Run VCS (33)	1m 30s
Run VCS (34)	28s
Run VCS (35)	1m 53s
Run VCS (36)	32s
Report VCS CI result	30s
	50m 19s

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