Collect test coverage for both unit & NixOS integration tests #530

Job	Run time
lint (golangci-lint)	59s
build (nix-snapshotter)	26s
build (image-hello)	22s
build (image-redis)	23s
build (image-redisWithShell)	27s
integration-test (snapshotter)	1m 33s
integration-test (kubernetes)	1m 55s
integration-test (k3s)	1m 35s
integration-test (k3s-external)	1m 39s
integration-test (k3s-rootless)	1m 42s
coverage-report	0s
	11m 1s

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