ci: docker: dist-various-1: Include RISC-V C compilers The compiler-builtins for RISC-V are missing some key functions, such as __bswapsi2 [1]. We can't just pull in the LLVM compiler-rt builtins as the rust-lang/rust distribution container doesn't have a C compiler [2]. This patch adds RISC-V C compilers to the CI Dockerfile as the first step towards enabling LLVM compiler-rt builtins for RISC-V Rust. 1: https://github.com/rust-lang/compiler-builtins/issues/350 2: https://github.com/rust-lang/compiler-builtins/commit/e4f46b91ca843297fc065e20f1591e4971ae608c