fix: forward $PATH to tmux popup in zsh (#3198)

The functionality to run a tmux popup has the problem that the command
runs as a child of the `tmux` *server* process, rather than the current
shell. One potential issue is that the `$PATH` is not forwarded to this
command, so the `atuin` command may not be found.

Fixes: https://github.com/atuinsh/atuin/issues/3182 (#3182)

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## Checks
- [x] I am happy for maintainers to push small adjustments to this PR,
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same thing

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