nixos/test-driver: do not break if the command writes to stderr

Capturing `stderr` as part of the return `output` could break existing tests.

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

@@ -582,9 +582,7 @@ class Machine:
 
         # 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)} | (base64 -w 0; echo)\n"
-        )
+        out_command = f"{timeout_str} bash -c {shlex.quote(command)} 2>/dev/null | (base64 -w 0; echo)\n"
 
         assert self.shell
         self.shell.send(out_command.encode())