Set sandbox architecture correctly in the bootstrap

Starting with bst 1.6 (1.5.1 to be exact), buildstream can pull artifacts built on a different architecture. This MR makes the necessary modifications to use this functionality (See also !2849 (comment 370937336)).

This allows us to drop the export and re_import plugins and use the artifact cache to transfer.

Here is an example of how to use this (by bootstrapping aarch64 on x86_64): 89e4e121

It's possible that this breaks riscv as it may be currently bypassing the sandbox arch setting in buildstream (bst thinks it is building x86_64 even though it is building riscv using qemu-user).

Closes #375 (closed), #1381 (closed)