Fix multi-platform builds for arm64

[negz]

Description of your changes

Currently we're somehow building a multi-platform image (i.e. manifest list) but the arm64 image has an amd64 bootstrapper binary in it. I'm honestly not exactly sure how this is happening.

In an attempt to fix it, I've copied the Makefile and Dockerfile from Crossplane v1.16 (before it moved to Earthly for image builds). I've verified that the arm64 image works locally - I think we need to actually build and push an image in CI to test multiplatform.

I have:

• [x] Read and followed Upbound's contribution process.
• [x] Run make reviewable to ensure this PR is ready for review.
• [ ] Added backport release-x.y labels to auto-backport this PR, as appropriate.

How has this code been tested

Just make build.all and then docker run build-f6f3baa5/uxp-bootstrapper-arm64 --help to verify it's built for the right architecture.

[negz]

For the record, this produced a working multi platform build.