k3s: use default buildGoModule [V2] #581

Job	Run time
build (nix-snapshotter)	1m 32s
lint (golangci-lint)	2s
build (image-hello)	1m 35s
build (image-redis)	1m 31s
build (image-redisWithShell)	1m 36s
integration-test (snapshotter)	4m 49s
integration-test (push-n-pull)	3m 54s
integration-test (kubernetes)	5m 56s
integration-test (k3s)	13m 16s
integration-test (k3s-external)	14m 49s
integration-test (k3s-rootless)	12m 53s
integration-test (gvisor)	4m 3s
coverage-report	44s
	1h 6m 40s

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