Variables for single command doesn't work with `exec`

[Harazi]

fish --version

fish, version 3.6.1

uname -a

Linux R3UB 6.4.12-arch1-1 #1 SMP PREEMPT_DYNAMIC Thu, 24 Aug 2023 00:38:14 +0000 x86_64 GNU/Linux

echo $TERM

tmux-256color

Overriding variables for a single command using var=val syntax doesn't work when the command to be executed is exec.

[ridiculousfish]

Good find, this should be fixed.