Remove override in the riscv64 image build job that make it use sid

Since yesterday (2024-08-12)[0], and after the gcc-14 transition to testing[1], the official debian docker images for riscv64 include trixie{-slim}, which is the current testing. We no longer need to rely on sid-slim to run the job that builds the riscv64 image.

[0] https://github.com/docker-library/official-images/pull/17350

[1] https://tracker.debian.org/news/1554202/gcc-14-1420-1-migrated-to-testing/