The `postinst` wrapper script into which these scripts are embedded as
`after_upgrade` and `after_install` functions are executed using Bourne
shell(`sh`), so we cannot use the Bash specific `[[ ]]` test or it fails:
```
/var/lib/dpkg/info/nimbus-beacon-node.postinst: 22: [[: not found
```

Signed-off-by: Jakub Sokołowski <[email protected]>