build(nix/config): configure opencode permissions #52

Job	Run time
check (aarch64-linux, ubuntu-24.04-arm)	34s
check (x86_64-linux, ubuntu-24.04)	33s
switch (x86_64-linux, ubuntu-24.04)	2m 49s
switch (aarch64-linux, ubuntu-24.04-arm)	3m 47s
	7m 43s

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