Make env in ImageConfig a map

Fields in CUE were renamed to the lowercase version of Dockerfile instructions.

There's now opportunity to make other fields simpler to use (e.g., healthcheck), this commit is focused on env.

Signed-off-by: Helder Correia <174525+helderco@users.noreply.github.com>

tests/tasks/dockerfile/image_config.cue

@@ -18,8 +18,8 @@ engine.#Plan & {
 				"""
 		} & {
 			config: {
-				Env: ["PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin", "test=foobar"]
-				Cmd: ["/bin/sh", "-c", "/test-cmd"]
+				env: test: "foobar"
+				cmd: ["/bin/sh", "-c", "/test-cmd"]
 			}
 		}
 	}