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`