Setting CC/CXX as Docker ENV vars caused LLVM's native tblgen
sub-build to also use the riscv64 cross-compiler, producing an
riscv64 binary that fails to run on the x86_64 host:
Exec format error: llvm-min-tblgen
Fix: pass the cross-compiler via CMAKE_C/CXX_COMPILER cmake flags
instead (cmake cache vars are not inherited by subprocess cmake
invocations, so LLVM's native build finds the host compiler).
Also pass CMAKE_SYSTEM_NAME=Linux/CMAKE_SYSTEM_PROCESSOR=riscv64 to
LLVM via WASI_SDK_CI_TOOLCHAIN_LLVM_CMAKE_ARGS so LLVM sets
CMAKE_CROSSCOMPILING=TRUE and triggers the native/target build split,
matching the pattern used by the macOS matrix entries.
Signed-off-by: Bruno Verachten <gounthar@gmail.com>