scripts/run_archiso.sh: disable PXE ROM

nl6720requested to merge
nl6720/archiso:no-PXE into master

ISO testing doesn't require the emulated firmware's PXE stack. For UEFI, it only disable iPXE in favor of EDK II network stack. https://github.com/tianocore/tianocore.github.io/wiki/EDKII-Network-Over-QEMU#verification-result

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