The default jdk was updated, but various applications were kept at
jdk8 so they could be updated one-by-one, testing that the update
does not break those packages.
I tested sweethome3d, and found out that while it runs fine with
the default jre, the build script of the current release does not
work on JDK9 or higher. This has already been fixed on trunk but
not yet released. Since this is specific to this package, I
moved the selection of 'jdk8' to inside the packages' .nix, with a
note so we don't forget to update to the default jdk as well when
a new version of sweethome3d is released.
b47bb058b8