Allow hidden tasks

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

tests/tasks/hidden/cue.mod/usr/testing.dagger.io/hidden/hidden.cue

@@ -0,0 +1,26 @@
+package hidden
+
+import (
+	"dagger.io/dagger/engine"
+)
+
+#Hidden: {
+	_pull: engine.#Pull & {
+		source: "alpine:3.15.0@sha256:e7d88de73db3d3fd9b2d63aa7f447a10fd0220b7cbf39803c803f2af9ba256b3"
+	}
+	_write: engine.#WriteFile & {
+		input:       _pull.output
+		path:        "/testing"
+		contents:    "1,2,3"
+		permissions: 700
+	}
+	_readfile: engine.#ReadFile & {
+		input: _write.output
+		path:  "/testing"
+	} & {
+		// assert result
+		contents: "1,2,3"
+	}
+	output:   _write.output
+	contents: _readfile.contents
+}