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

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

stdlib/europa/dagger/engine/fs.cue

@@ -49,11 +49,7 @@ package engine
 }
 
 // Produce an empty directory
-#Scratch: {
-	$dagger: task: _name: "Scratch"
-
-	output: #FS
-}
+#Scratch: {$dagger: fs: _id: null}
 
 // Copy files from one FS tree to another
 #Copy: {

stdlib/europa/dagger/engine/types.cue

@@ -7,7 +7,7 @@ package engine
 //  - A directory containing binary artifacts
 // Rule of thumb: if it fits in a tar archive, it fits in a #FS.
 #FS: {
-	$dagger: fs: _id: string
+	$dagger: fs: _id: string | null
 }
 
 // A reference to an external secret, for example:

stdlib/europa/dagger/utils.cue

@@ -16,12 +16,9 @@ import (
 	// Subdirectory tree
 	output: #FS & copy.output
 
-	// Base image
-	scratch: engine.#Scratch
-
 	// Copy action
 	copy: engine.#Copy & {
-		"input": scratch.output
+		"input": engine.#Scratch
 		source: {
 			root:   input
 			"path": path