internal/pkg: place ephemeral upperdir in tmp

This enables the use of directories made writable this way as scratch space.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/pkg/exec.go

@@ -7,6 +7,7 @@ import (
 	"os"
 	"runtime"
 	"slices"
+	"strconv"
 	"syscall"
 	"time"
 
@@ -325,7 +326,18 @@ func (a *execArtifact) cure(c *CureContext, hostNet bool) (err error) {
 		}
 
 		if a.paths[i].W {
-			z.OverlayEphemeral(b.dst, b.src...)
+			tempUpper, tempWork := temp.Append(
+				".upper", strconv.Itoa(i),
+			), temp.Append(
+				".work", strconv.Itoa(i),
+			)
+			if err = os.MkdirAll(tempUpper.String(), 0700); err != nil {
+				return
+			}
+			if err = os.MkdirAll(tempWork.String(), 0700); err != nil {
+				return
+			}
+			z.Overlay(b.dst, tempUpper, tempWork, b.src...)
 		} else if len(b.src) == 1 {
 			z.Bind(b.src[0], b.dst, 0)
 		} else {