Handle non-script grub2-install
Handle non-script grub2-install

grub2-install has been re-written [1] so isn't always a shell script,
so just exec it directly.  Also use $() as that follows the rest of
the file.

[1] http://git.savannah.gnu.org/cgit/grub.git/commit?id=cd46aa6cefabd7b0fcb15cd614577e1ab8c7a841

Change-Id: Ib4fd7c6854fb485c0da6a79babf4a3d45a8a0f2c

NaohiroTamura/diskimage-builder,varunarya10/diskimage-builder,zioproto/diskimage-builder,rdo-management/diskimage-builder,varunarya10/diskimage-builder,zioproto/diskimage-builder,CSC-IT-Center-for-Science/diskimage-builder,switch-ch/diskimage-builder,eantyshev/diskimage-builder,NaohiroTamura/diskimage-builder,switch-ch/diskimage-builder,takahashinobuyuki/diskimage-builder,JioCloud/diskimage-builder,openstack/diskimage-builder,openstack/diskimage-builder,eantyshev/diskimage-builder,takahashinobuyuki/diskimage-builder,CSC-IT-Center-for-Science/diskimage-builder,rdo-management/diskimage-builder,JioCloud/diskimage-builder