nixos/nexus: increase disk size of VM test to 8GB

Nexus increased their default minimum disk space requirement to 4GB:

```
com.orientechnologies.orient.core.exception.OLowDiskSpaceException: Error occurred while executing a
write operation to database 'OSystem' due to limited free space on the disk (1823 MB). The database
is now working in read-only mode. Please close the database (or stop OrientDB), make room on your hard
drive and then reopen the database. The minimal required space is 4096 MB. Required space is now
set to 4096MB (you can change it by setting parameter storage.diskCache.diskFreeSpaceLimit) .
server# [   72.560866] zqnav3mg7m6ixvdcacgj7p5ibijpibx5-unit-script-nexus-start[627]:   DB name="OSystem"
```

Including the rest on the VM 8GB should be the most suitable solution.
As the installer test also takes 8GB of disk size this should still be
in an acceptable range.