riscv64 builds

[totaam]

Added some riscv64 variants to the Debian build envs in Xpra-org/repo-build-scripts@e095f4a
The trixie container images aren't available yet but the sid ones are.

The build scripts worked on the first try for the xpra packages and we now have riscv64 beta builds:
https://xpra.org/beta/sid/main/binary-riscv64/

[totaam]

The Fedora 38 riscv64 build env can be setup by manually pulling this base image:

buildah from --arch riscv64  --name fedora-38-riscv64-repo-build imbearchild/fedora-rv64

Then modifying the setup script to not delete this existing bare bones repo build image (this is only needed until the official images are available).
Plus this minor change: Xpra-org/repo-build-scripts@47bef80
Disable x264 because this MR is pending: Add Risc-V 64 bit to config.guess (debian sid must have already merged it?)

So we now have some Fedora 38 riscv64 packages too:
https://xpra.org/beta/Fedora/38/riscv64/
But not for xpra itself which crashes with messages like this one:

creating build/lib.linux-riscv64-cpython-311/xpra/platform/posix: Resource temporarily unavailable

Apparently, something not dealing with EAGAIN? In qemu or podman?
Hopefully, this will be ironed out.