Support using binaries for a different architecture than the host #19
Description
On other CI systems, you can set the architecture to x86 or x64 and the corresponding Julia binaries will be installed regardless of the host's architecture. This permits testing 32-bit Julia builds on 64-bit hosts. However, it may be the case that Cirrus supports 32-bit x86 and/or ARMv7 hosts, in which case this would be unnecessary since folks can just use an image that matches the target architecture.