Smaller, cleaner test

Signed-off-by: Joel Longtine <joel@dagger.io>

tests/tasks/scratch/scratch_build_scratch.cue

@@ -6,18 +6,10 @@ import (
 
 engine.#Plan & {
 	actions: {
-		write: engine.#WriteFile & {
-			input:       engine.#Scratch
-			path:        "/.dockerignore"
-			contents:    "Dockerfile"
-			permissions: 700
-		}
 
 		build: engine.#Build & {
-			source: write.output
-			dockerfile: contents: """
-				FROM scratch
-				"""
+			source: engine.#Scratch
+			dockerfile: contents: "FROM scratch"
 			// Assert that output is engine.#Scratch
 			output: engine.#Scratch
 		}