make `vt-tv` work with `fmt` v11

[cz4rs]

Spotted in DARMA-tasking/vt#2399:

/vt/lib/vt-tv/tests/unit/render/test_render.cc:265:14: error: 'join' is not a member of 'fmt'
  265 |         fmt::join(cmd_vars, " "),
      |              ^~~~

Full logs: https://dev.azure.com/DARMA-tasking/DARMA/_build/results?buildId=65446&view=logs&j=3dc8fd7e-4368-5a92-293e-d53cefc8c4b3&t=28db5144-7e5d-5c90-2820-8676d630d9d2.

fmt version used: 11.1.3.