Wrong shell is used when executing on_project_start commands

[MLNW]

Describe the bug
When adding commands to the on_project_start block the sh shell is used instead of the default shell, in this case:

❯ echo $SHELL
/usr/bin/zsh

To Reproduce

1. Create a new tmuxinator project
2. Add following block:

on_project_start:
    - ps -p $$
    - echo $SHELL

3. Start the project
4. See in the output that sh is used instead of the default shell

Expected behavior
The default shell should be used when executing on_project_* commands.

Environment

• tmuxinator Version: tmuxinator 3.0.1
• tmux Version: tmux 3.1c
• Ruby Version: ruby 2.7.2p137 (2020-10-01 revision 5445e04352) [x86_64-linux-gnu]
• OS: Ubuntu 21.04 running in WSL2

Additional information
I'm trying to activate the conda environment of the project, for which the executing shell must be initialized. sh is not zsh is.

[MLNW]

To work around my issue it is possible to activate the conda environment via the pre_window hook.

I still believe on_project_start should use the default shell.

[franalbani]

The same happens with on_project_first_start.