revert input dir management to llb.Copy to fix cache for yarn source pkg

Signed-off-by: Sam Alba <sam.alba@gmail.com>

environment/pipeline.go

@@ -342,9 +342,20 @@ func (p *Pipeline) Local(ctx context.Context, op *compiler.Value, st llb.State)
 		opts = append(opts, llb.ExcludePatterns(excludePatterns))
 	}
 
-	return llb.Local(
+	// FIXME: Remove the `Copy` and use `Local` directly.
-		dir,
+	//
-		opts...,
+	// Copy'ing is a costly operation which should be unnecessary.
+	// However, using llb.Local directly breaks caching sometimes for unknown reasons.
+	return st.File(
+		llb.Copy(
+			llb.Local(
+				dir,
+				opts...,
+			),
+			"/",
+			"/",
+		),
+		llb.WithCustomName(p.vertexNamef("Local %s [copy]", dir)),
 	), nil
 }