checkpoint do not modify XDG_RUNTIME_DIR

We should not modify the XDG_RUNTIME_DIR env value during runtime of
libpod, this can cause hard to find bugs. Only set it for the OCI
runtime, this matches the other commands such as start, stop, kill...

[NO NEW TESTS NEEDED]

Signed-off-by: Paul Holzinger <pholzing@redhat.com>

utils/utils.go

@@ -43,9 +43,7 @@ func ExecCmdWithStdStreams(stdin io.Reader, stdout, stderr io.Writer, env []stri
 	cmd.Stdin = stdin
 	cmd.Stdout = stdout
 	cmd.Stderr = stderr
-	if env != nil {
-		cmd.Env = env
-	}
+	cmd.Env = env
 
 	err := cmd.Run()
 	if err != nil {