ci(runner): split build across per-arch runners

[andrewazores]

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See cryostatio/cryostat#786
Related to cryostatio/cryostat-operator#611

Description of the change:

Splits up the CI job for building containers, so that each arch (amd64 and arm64) for the multiarch manifest is built in parallel, and on a native-arch runner rather than cross-compiling using qemu.

Motivation for the change:

Very significantly faster build. Sequentially building the two archs on an amd64 runner usually takes about an hour total, but this time is heavily dominated by the emulation slowdown of cross-compilation running under qemu. With this change each build takes a bit over 10 minutes, and since they are done in parallel, the total CI completion time goes from over an hour to just over 10 minutes.

Sample runs:
Before: https://github.com/cryostatio/cryostat-openshift-console-plugin/actions/runs/13016113128
After: https://github.com/andrewazores/cryostat-openshift-console-plugin/actions/runs/13016583361 (result - https://quay.io/repository/andrewazores/cryostat-openshift-console-plugin?tab=tags)