Allow qemu-user builds (so target arch can be different than host arch)
Context
This is needed to for example build for RISC-V in an x86_64 machine (which bst1 supports fine)
This is blocking fdsdk switch to bst2, see freedesktop-sdk/freedesktop-sdk#1383 (closed)
Acceptance Criteria
It's possible to use buildstream to build for RISC-V in qemu in a x86_64 (or other arch)