Use faster nix installer action and cache for CI #5075

Job	Run time
Test Frontend	4m 51s
Test + Coverage	4m 2s
Build nix environment on MacOS	6m 25s
Formatting	2m 29s
Linter	2m 55s
Test (shuffled order)	3m 29s
MyPy	2m 35s
Test if testdata is up to date	3m 26s
Test Typescript	2m 34s
Build wheel	3m 10s
Install wheel with system Python	13s
Backup process	1m 44s
Test (shuffled order)	1m 42s
Test + Coverage	2m 29s
Build nix environment on MacOS	1m 53s
Test Frontend	5m 20s
Formatting	49s
Test Typescript	58s
MyPy	58s
Build wheel	1m 1s
Test if testdata is up to date	1m 29s
Linter	1m 29s
Backup process	1m 29s
Install wheel with system Python	16s
	57m 46s

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