Streamline --arch arg with --os

[am11]

Currently, this works:

src/tests/build.sh --os $os --$arch

but neither this symmetrical variant works:

src/tests/build.sh --$os --$arch

nor this:

src/tests/build.sh --os $os --arch $arch

PR adds --arch in addition to the existing --$arch to avoid breaking local workflows.

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[jkotas]

Thanks