Build debian Matrix.org packages for aarch64

[szclsya]

Description:

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

Previously (matrix-org/synapse#5685) it seems possible to use deb-src to work around the limitation but this method no longer works.

[erikjohnston]

I don't think we have anything against building for aarch64, but unfortunately it is quite low on our priority list.

[ZicPL]

I think RISC-V 64-bit architecture should be on that list, even if it was low priority. Element (matrix.org) could participate in solidarity for another open source project✌️