refactor(home.nix): add mkXdgConfigDirs helper #32

Job	Run time
check (aarch64-linux, ubuntu-24.04-arm)	37s
check (x86_64-linux, ubuntu-24.04)	35s
switch (aarch64-linux, ubuntu-24.04-arm)	1m 59s
switch (x86_64-linux, ubuntu-24.04)	2m 6s
	5m 17s

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