Fish shell 3.1.0 has a regression

[David Kowis]

```
~/.config/fish/functions/fish_prompt.fish (line 281): Expected a string, but ins tead found a '&'
  if which rvm-prompt >/dev/null ^&1
```

https://github.com/fish-shell/fish-shell/issues/6614

<email address hidden> /home/dkowis > lsb_release -rd
Description: Ubuntu 20.04 LTS
Release: 20.04

<email address hidden> /home/dkowis > apt-cache policy fish
fish:
  Installed: 3.1.0-1.2
  Candidate: 3.1.0-1.2
  Version table:
 *** 3.1.0-1.2 500
        500 http://us.archive.ubuntu.com/ubuntu focal/universe amd64 Packages
        100 /var/lib/dpkg/status

I expected to have a working fish shell, but unfortunately they haven't released 3.1.1

Shipping a broken fish shell in latest ubuntu isn't a good idea tho.

[Kai Kasurinen (kai-kasurinen)]

Thank you for taking the time to report this bug and helping to make Ubuntu better. However, I am closing it because the bug has been fixed in the latest development version of Ubuntu.

If you need a fix for the bug in previous versions of Ubuntu, please perform as much as possible of the SRU Procedure [1] to bring the need to a developer's attention.

[1]: https://wiki.ubuntu.com/StableReleaseUpdates#Procedure