Remove riscv64 from build platforms

[das7pad]

Description of the change

I'm hoping this will unblock the release of jammy-1.0.2.

The docker image ubuntu:22.04 is not available for riscv64, which limits options for providing support for it here.

------
 > [linux/riscv64 internal] load metadata for docker.io/library/ubuntu:22.04:
------
Dockerfile:2
--------------------
   1 |     ARG BASE_IMAGE=ubuntu:22.04
   2 | >>> FROM $BASE_IMAGE
   3 |     
   4 |     ARG QEMU_ARCH
--------------------
ERROR: failed to solve: ubuntu:22.04: no match for platform in manifest: not found
Error: buildx failed with: ERROR: failed to solve: ubuntu:22.04: no match for platform in manifest: not found

Benefits

Unblocks release?

Possible drawbacks

cc @olof-nord who added this in 2021 via #604, back then the release was focal.

Applicable issues

#632

Additional information

[samip5]

This breaks riscv64, but the newer ubuntu is not available for it so YOLO.

[olof-nord]

You are the ones in charge of the project, no questions asked.
I do see this as a step backwards though, as risc-v is supported by Ubuntu 22.04.3 as well as 23.10.

https://ubuntu.com/download/risc-v

For risc-v docker Ubuntu images, I use https://hub.docker.com/r/riscv64/ubuntu at the moment.

[das7pad]

I've opened #634 with an attempt to bring back support for riscv64. Cross-posting:

It is worth noting that the riscv64/ubuntu:22.04 image was last built in December 2022, so more than a year ago. It does not look supported anymore.