image-tests: checkpoint build root

Most images share a build root, checkpoint the build root to speed up tests
at the potential cost of some disk space (in the cases the build roots are not the same).

cmd/osbuild-image-tests/constants/constants.go

@@ -9,6 +9,7 @@ func GetOsbuildCommand(store, outputDirectory string) *exec.Cmd {
 		"osbuild",
 		"--store", store,
 		"--output-directory", outputDirectory,
+		"--checkpoint", "build",
 		"--json",
 		"-",
 	)