Revert multi-arch build support to avoid QEMU segfaults

[MikaKerman]

Reverts elementary-data/elementary#1701

This PR reverts the changes that added support for the linux/arm64 architecture in our Docker builds. We’ve been encountering recurring segmentation faults under QEMU when installing certain packages (specifically libsasl2-dev), causing the arm64 build to fail.

Since we do not strictly need an arm64 image at this time, this revert removes the multi-arch workflow steps and returns us to a straightforward amd64-only build. By reverting these changes, we avoid the QEMU user-mode emulation crashes and restore reliable builds.

We can reconsider arm64 support in the future if it becomes necessary or if we move to a native arm64 builder.

[github-actions[bot]]

👋 @MikaKerman Thank you for raising your pull request. Please make sure to add tests and document all user-facing changes. You can do this by editing the docs files in this pull request.

[MikaKerman]

Hey @JanDintel, We had to revert the PR due to QEMU segfaults on arm64 that occur whenever certain packages trigger a libc reconfiguration. We added a new image build workflow, allowing testing of the build before the package release process.