cmds/switch_root: complete switch from syscall to x/sys/unix
cmds/switch_root: complete switch from syscall to x/sys/unix

Follow up for commit cc9ad413aab2, where unix.Exec was not there yet.

hugelgupf/u-root,hugelgupf/u-root,rminnich/u-root,rminnich/u-root,GanShun/u-root,hugelgupf/u-root,GanShun/u-root,rminnich/u-root,GanShun/u-root,u-root/u-root,u-root/u-root,u-root/u-root,rminnich/u-root,hugelgupf/u-root