Adding shellcheck static analysis in CI #612

Job	Run time
lint (golangci-lint)	4s
build (nix-snapshotter)	1m 51s
lint (shellcheck)	2s
build (image-hello)	1m 39s
build (image-redis)	2m 10s
build (image-redisWithShell)	2m 10s
integration-test (snapshotter)	3m 40s
integration-test (push-n-pull)	2m 58s
integration-test (kubernetes)	3m 57s
integration-test (k3s)	11m 55s
integration-test (k3s-external)	13m 5s
integration-test (k3s-rootless)	11m 49s
integration-test (gvisor)	2m 48s
coverage-report	3m 0s
	1h 1m 8s

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