sapphire-localnet: Run CI on Mac too

[matevz]

We had a report that the Mac version of the sapphire- and emerald-localnet images don't spin up on Apple M chips even though --platform linux/x86_64 flag was correctly passed. Add the Mac flavor to the runs-on github action directive to make sure it runs on non-x86 architectures as well.

cc @lubej

[aefhm]

I don't think is doable yet? #677 (comment)

[aefhm]

Can confirm sapphire-localnet works on M4.

[aefhm]

We could also suggest that Mac M chip devs should always set the target platform.

# Specify target platform for M chip Macs
export DOCKER_DEFAULT_PLATFORM = linux/amd64