Build debian Matrix.org packages for aarch64

**Description:**

Currently matrix.org's debian repository only contains packages for amd64 and i386 (see [repository index for bookworm](https://packages.matrix.org/debian/dists/bookworm/main/)) It would be useful for Raspberry Pi and aarch64 vps users (i.e Amphere Altra) if such repository also contains packages for aarch64.

Previously (https://github.com/matrix-org/synapse/issues/5685) it seems possible to use `deb-src` to work around the limitation but this method no longer works.