Fix make test on non-x86_64 arches

[cfergeau]

The tests in test can only be built on x86_64 as test/fcos.go uses a getFCOSDownload method which is only defined in test/fcos_amd64.go
This means go test ./test will only be buildable on an x86_64 machine.
This is problematic as make test in .github/workflows/go.yml is run on a macos node which can be arm64 or amd64, and thus the action will fail depending on which node is picked.

[cfergeau]

macos-latest github runners are switching to M1 which do not support nested virt, which is going to be another issue for our tests. https://docs.github.com/en/actions/using-github-hosted-runners/about-github-hosted-runners/about-github-hosted-runners#standard-github-hosted-runners-for-public-repositories
The ubuntu runners may support nested virt now though actions/runner-images#7541

[cfergeau]

I created #358 for this.

[cfergeau]

This was fixed by #362