[ re #155 ] Minor simplifications following suggestions by @omelkonian #258

Job	Run time
Test building inside a nix shell	1m 24s
Compile agda2hs with nix (agda2hs)	57s
Typecheck with Agda with nix (base-lib)	1m 4s
Typecheck with Agda with nix (containers-lib)	1m 17s
	4m 42s

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