k3s: use default buildGoModule [V2] #582

Job	Run time
build (nix-snapshotter)	1m 26s
lint (golangci-lint)	7s
build (image-hello)	1m 35s
build (image-redis)	1m 36s
build (image-redisWithShell)	1m 33s
integration-test (snapshotter)	4m 45s
integration-test (push-n-pull)	4m 16s
integration-test (kubernetes)	4m 49s
integration-test (k3s)	12m 52s
integration-test (k3s-external)	14m 55s
integration-test (k3s-rootless)	13m 17s
integration-test (gvisor)	4m 58s
coverage-report	49s
	1h 6m 58s

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