Use custom `installCommand` for `cachix-action` #667

Job	Run time
Build remote home configuration	2m 52s
Build devShell	16m 4s
Check flake	3m 52s
Build system (eMac, macos-14)	5m 28s
Build system (st-eturkeltaub2, macos-14)	4m 51s
	33m 7s

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