internal/pkg: expose cure whence

Useful for cure determinism validation of KindExec.

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

internal/pkg/file_test.go

@@ -18,7 +18,7 @@ func TestFile(t *testing.T) {
 				{"short", pkg.NewFile("null", []byte{0}), base.Append(
 					"identifier",
 					"3376ALA7hIUm2LbzH2fDvRezgzod1eTK_G6XjyOgbM2u-6swvkFaF0BOwSl_juBi",
-				), want, nil},
+				), want, pkg.WNew, nil},
 			})
 		}, expectsFS{
 			".": {Mode: fs.ModeDir | 0700},