Preserve repos between stage2 and stage3

This will add to the image size and push/pull times, but it saves from
having to do an eix-sync for each stage3 build and it lets the image
preserve the local crossdev repo which can't easily be regenerated.
8 jobs for main in 474 minutes and 41 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #859
nest
Build beagleboneblack-server

01:44:12

passed #886
nest
Build haswell-server

00:25:27

passed #855
nest
Build haswell-workstation

01:08:51

passed #856
nest
Build pinebookpro-workstation

04:31:51

passed #857
nest
Build raspberrypi-workstation

04:36:08

passed #858
nest
Build sopine-server

02:17:17

passed #854
nest
Build haswell-server

00:26:54

canceled #885
nest
Build haswell-server

00:05:59