Closes#46551
Zed was using the `terminal.shell` setting for some actions unrelated to
the terminal GUI, like environment capture, ACP agent startup, context
server startup, and vim `:!`, and so if a user set it to `tmux` or
`nushell`, those paths would fail.
This PR ensures paths unrelated to Zed's terminal use the system path
instead.
Manual testing:
Set `terminal.shell` to `tmux` or any another non-bash executable.
Starting an external agent thread shouldn't break.
Release Notes:
- Fixed ACP agent and other breakage when `terminal.shell` was set to a
non-shell program like `tmux`