Command substitution leaks processes if they expect input (command substitutions run in their own process group)

Example: `set -l var (cat)`

Expected: `$var` contains value of user input.

Actual: `$var` contains an empty string, and if you run `jobs`, a stopped but not reaped `cat` process lingers unless you run `kill -9` on it.

I understand that in this case, I could use `read`, but this error would still be a thing.
