Fix fetching artifacts when sudo changes directory

The artifact sending logic is basically:

```sh
cd directory
sudo tar ...
```

The problem is that sudo may have done something that changes the directory.
I've observed this while testing on Ubuntu 24.10. Instead of fiddling with more
fringe options around sudo, pass -C or --directory= to tar, so that all the
files are referenced relative to the task directory.

Signed-off-by: Zygmunt Krynicki <[email protected]>

spread/client.go

@@ -657,7 +657,7 @@ func (c *Client) RecvTar(packDir string, include []string, tar io.Writer) error
 	var stderr safeBuffer
 	session.Stdout = tar
 	session.Stderr = &stderr
-	cmd := fmt.Sprintf(`cd '%s' && %s/bin/tar cJ --sort=name --ignore-failed-read -- %s`, packDir, c.sudo(), strings.Join(args, " "))
+	cmd := fmt.Sprintf(`%s/bin/tar -C %q -cJ --sort=name --ignore-failed-read -- %s`, c.sudo(), packDir, strings.Join(args, " "))
 	err = c.runCommand(session, cmd, nil, &stderr)
 	if err != nil {
 		return outputErr(stderr.Bytes(), err)