tests: add unpack to scratch export test

The original commit (fddd129) that
added this test did so to check that empty layers wouldn't be unpacked
which would cause a nil de-reference.

However, `unpack` wasn't added to the export test, so the test wasn't
actually testing the changed behavior.

Signed-off-by: Justin Chadwell <[email protected]>

Author: jedevc

client/client_test.go

@@ -2108,6 +2108,7 @@ func testBuildExportScratch(t *testing.T, sb integration.Sandbox) {
 				Attrs: map[string]string{
 					"name":        target,
 					"push":        "true",
+					"unpack":      "true",
 					"compression": "uncompressed",
 				},
 			},