test-driver: revert stderr nullpipe

Removes 2>/dev/null which re-adds stderr output breaking execute

nixos/lib/test-driver/test_driver/machine.py

···
 
         # While sh is bash on NixOS, this is not the case for every distro.
         # We explicitly call bash here to allow for the driver to boot other distros as well.
-        out_command = f"{timeout_str} bash -c {shlex.quote(command)} 2>/dev/null | (base64 -w 0; echo)\n"
+        out_command = (
+            f"{timeout_str} bash -c {shlex.quote(command)} | (base64 -w 0; echo)\n"
+        )
 
         assert self.shell
         self.shell.send(out_command.encode())