Interactive command substitution does not work

This was discovered while testing and developing https://github.com/ajeetdsouza/zoxide/pull/43. If you try to assign an interactive command (such as `fzf`, which `zoxide query -i` calls after some preprocessing) to a variable, nothing happens.

### STR

1. Grab a usable copy of `fzf` or some other similarly interactive command
2. `var="$(fzf)"`
3. Note the lack of interactivity, as well as the empty `var`

In `dash` and `bash --posix`, this works as expected, with `var` being set to e.g. `Cargo.lock` (or whatever file was picked).

(Feel free to reword the title if it's misleading; I'm not exactly a shell expert, so I may have used incorrect terminology.)

EDIT: Pictures.

#### `mrsh`

#### `dash`

#### `bash --posix`