Make sure apt caches are up2date before trying to install aptitude

Otherwise we might run into:

| [...]
| 08:54:38 preserving etc/apt/sources.list.d/grml-stable.list
| 08:54:39 OK
| 08:54:39 aptitude binary is not available, installing to make sure further FAI actions continue to work
| 08:54:39 Reading package lists...
| 08:54:42 Building dependency tree...
| 08:54:42 Package aptitude is not available, but is referred to by another package.
| 08:54:42 This may mean that the package is missing, has been obsoleted, or
| 08:54:42 is only available from another source
| 08:54:42
| 08:54:42 E: Package 'aptitude' has no installation candidate
| 08:54:42 updatebase.GRMLBASE  FAILED with exit code 100.
| 08:54:42 Calling task_updatebase

As nowadays - with Debian/jessieu - aptitude no longer is in
a base debootstrap chroot.