Make engine.#Scratch a specialization of #FS, rather than a task

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

tests/cue.mod/module.cue

@@ -0,0 +1 @@
+module: ""

tests/cue.mod/pkg/.gitignore

@@ -0,0 +1,3 @@
+# generated by dagger
+alpha.dagger.io
+dagger.lock

tests/plan/outputs/outputs.cue

@@ -4,10 +4,8 @@ import "alpha.dagger.io/europa/dagger/engine"
 
 engine.#Plan & {
 	actions: {
-		scratch: engine.#Scratch
-
 		data: engine.#WriteFile & {
-			input:       scratch.output
+			input:       engine.#Scratch
 			path:        "/test"
 			permissions: 0o600
 			contents:    "foobar"

tests/tasks/scratch/scratch.cue

@@ -10,13 +10,11 @@ engine.#Plan & {
 			source: "alpine:3.15.0@sha256:e7d88de73db3d3fd9b2d63aa7f447a10fd0220b7cbf39803c803f2af9ba256b3"
 		}
 
-		scratch: engine.#Scratch
-
 		exec: engine.#Exec & {
 			input: image.output
 			mounts: fs: {
 				dest:     "/scratch"
-				contents: scratch.output
+				contents: engine.#Scratch
 			}
 			workdir: "/"
 			args: [