Add snap paths to fish config - otherwise snaps aren't available via desktop menu

[Reuben Firmin]

For ubuntu packaging, please add snap paths to the fish config. Otherwise, users who switch to fish lose any installed snaps from the desktop menu. Please see:

https://askubuntu.com/questions/990071/why-do-snap-packages-disappear-when-using-the-fish-shell

Fix:

When installing fish, ensure the default fish config contains:

```
set PATH /var/lib/snapd/snap/bin $PATH
set XDG_DATA_DIRS /var/lib/snapd/desktop/:$XDG_DATA_DIRS
```

ProblemType: Bug
DistroRelease: Ubuntu 21.10
Package: fish 3.1.2-3ubuntu1
ProcVersionSignature: Ubuntu 5.13.0-22.22-generic 5.13.19
Uname: Linux 5.13.0-22-generic x86_64
NonfreeKernelModules: nvidia_modeset nvidia
ApportVersion: 2.20.11-0ubuntu71
Architecture: amd64
CasperMD5CheckResult: pass
CurrentDesktop: KDE
Date: Sat Dec 18 04:50:40 2021
InstallationDate: Installed on 2021-12-16 (1 days ago)
InstallationMedia: Kubuntu 21.10 "Impish Indri" - Release amd64 (20211012)
SourcePackage: fish
UpgradeStatus: No upgrade log present (probably fresh install)