fish: 3.0.2 -> 3.1.0

fish-shell 3.1.0 was just released today with many niceties (including
the ability to have bash-like temporary env vars e.g. `VAR="var1"
command` instead of needing to use `env VAR="var1" command`). To see the
full list of changes, please visit
https://github.com/fish-shell/fish-shell/releases/tag/3.1.0.