ci: add neovim 0.12 #771

Job	Run time
luals-check (nightly, 5.1)	40s
luals-check (0.12, luajit-master)	42s
luals-check (0.12, 5.1)	36s
luals-check (nightly, luajit-master)	51s
	2m 49s

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