Simplify setting arch for the final stage

[meeDamian]

I think you could simplify that a lot with a build arg.

See:

• https://github.com/lukechilds/docker-bitcoind/blob/abd6f87ca84e24061e2ee81375db3ce6914bb11a/.github/workflows/ci.yml#L37
• https://github.com/lukechilds/docker-bitcoind/blob/abd6f87ca84e24061e2ee81375db3ce6914bb11a/Dockerfile#L4

It's one extra command in the build system to register QEMU and then a build arg to specify the architecture.

How sure are you that qemu is able to do that perfectly for every single arbitrary assembly instruction?

It's used for emulation by the Android SDK so I'd have to imagine it's pretty accurate but I really don't know.

Originally posted by @lukechilds in #48

[meeDamian]

Done