Snap package complains about "update check failed"

**Describe the bug**

After every time I run `taskbook`, I get a big warning printed:

```
┌────────────────────────────────────────────────────────┐
│ taskbook update check failed │
│ Try running with sudo or get access │
│ to the local update config store via │
│ sudo chown -R $USER:$(id -gn $USER) /home/josh/.config │
└────────────────────────────────────────────────────────┘
```

**To Reproduce**

```console
$ snap install taskbook
$ taskbook
```

**Expected behavior**

Taskbook shouldn't try to update itself when it's a snap – the whole point of snaps is that they're immutable, it's expected that it can't overwrite itself.

**Technical Info**
- OS: Ubuntu 18.04
- Node.js Version: whatever's in the snap
- Taskbook Version: 0.2.0

Reproduce the warning with `snap install taskbook` followed by `taskbook` on Ubuntu 18.04, then trace the update-check path used by the snap-installed CLI. Done means the snap no longer attempts a self-update or prints the update-check warning, while the existing behavior for other installations remains unchanged.

Written by the indexing model from the issue text.