dav1d-debian-unstable: Switch to cross-compiling libc instead of multi-arch

Since riscv64 is a relatively new architecture, there are still some
growing pains. Multi-arch is currently not fully possible due to version
mismatches wrt binNMU [1].

Since we only depend on libc for the main functionality,
libc6-dev-riscv64-cross should suffice.
As a downside, we now need to specify the library path for qemu-user
with `-L /usr/riscv64-linux-gnu/` or the `QEMU_LD_PREFIX` env var.

[1] https://wiki.debian.org/binNMU

dav1d-debian-unstable/Dockerfile

-FROM debian:sid-20230502-slim
+FROM debian:sid-20231030-slim
 
 MAINTAINER VideoLAN roots <roots@videolan.org>
 
@@ -20,15 +20,8 @@ RUN groupadd --gid ${VIDEOLAN_UID} videolan && \
         lftp ca-certificates curl git build-essential \
         nasm clang libclang-rt-dev mold meson ninja-build gcovr \
         wine wine64 procps doxygen graphviz libsdl2-dev ripgrep \
-        debian-ports-archive-keyring zstd && \
-    sed -i -e '$aArchitectures-Remove: riscv64' /etc/apt/sources.list.d/debian.sources && \
-    sed -e '/^URIs/c URIs: http://deb.debian.org/debian-ports' \
-        -e '/^Signed-By/c Signed-By: /usr/share/keyrings/debian-ports-archive-keyring.gpg' \
-        -e '/^Architectures/c Architectures: riscv64' \
-        /etc/apt/sources.list.d/debian.sources \
-        > /etc/apt/sources.list.d/debian-ports.sources && \
+        zstd && \
     dpkg --add-architecture i386 && \
-    dpkg --add-architecture riscv64 && \
     apt-get update && \
     apt-get install --no-install-suggests --no-install-recommends -y \
         gcc-mingw-w64-x86-64 g++-mingw-w64-x86-64 \
@@ -36,7 +29,7 @@ RUN groupadd --gid ${VIDEOLAN_UID} videolan && \
         gcc-multilib g++-multilib \
         libc6-dev:i386 libgcc-12-dev:i386 wine32 qemu-user && \
     apt-get install --no-install-suggests --no-install-recommends -y \
-        gcc-riscv64-linux-gnu g++-riscv64-linux-gnu libc6-dev:riscv64 && \
+        gcc-riscv64-linux-gnu g++-riscv64-linux-gnu libc6-dev-riscv64-cross && \
     apt-get clean -y && rm -rf /var/lib/apt/lists/*
 
 USER videolan