fish: Fixed tab completion with sudo

When hitting tab on a sudo command in fish, I get the following error message:
```
set: Warning: path component /sbin may not be valid in PATH.
set: No such file or directory
set: Warning: path component /usr/sbin may not be valid in PATH.
set: No such file or directory
```
This is because we don't have /sbin and /usr/sbin. I replaced them with more suitable values.