riscv-64 Docker build: unblock Ubuntu 26.04 by pinning host compiler and unsafe-configure override

[Copilot]

PR #360 moved the riscv-64 test artifact image to Ubuntu 26.04, but the Buildroot stage failed in CI: first on host-tar root checks, then on host-m4 compilation under the default Ubuntu 26.04 host toolchain. This update aligns the container environment with Buildroot’s expectations for this path.

• Buildroot configure behavior

  • Added FORCE_UNSAFE_CONFIGURE=1 before the Buildroot make step to bypass host-tar’s root configure guard inside the Docker build context.

• Host toolchain stabilization on Ubuntu 26.04

  • Installed gcc-13 / g++-13.
  • Exported HOSTCC and HOSTCXX to force Buildroot host packages (including host-m4) to compile with GCC 13 instead of the Ubuntu 26.04 default compiler set.

• riscv-64 Dockerfile updates

  • Kept the Ubuntu 26.04 base-image bump and layered the CI compatibility changes in the same file:
    • /tests/rsrc/riscv-64/Dockerfile

ENV FORCE_UNSAFE_CONFIGURE=1
ENV HOSTCC=/usr/bin/gcc-13
ENV HOSTCXX=/usr/bin/g++-13
RUN make BR2_EXTERNAL=/test/test-kernel-modules/ simics_simple_riscv_defconfig && \
    make && \
    ...