Isolate whoami unconfigured test from local dotfiles

[thibaudcolas]

Summary

• test_whoami_unconfigured_exit_2 failed on machines with a real ~/.wagtail-cli.toml: load_config picked up the local token, so the CLI appeared configured and hit the fake URL, exiting 3 (NetworkError) instead of the expected 2 (UsageError).
• The test now monkeypatches wagtail_cli.config._user_dotfile and _project_dotfile to a tmp path, matching the isolation pattern already used by test_init_writes_dotfile.

Test instructions

just test and just lint pass locally (287 passed, 1 skipped).