ci: fix riscv64 LLVM cross-compilation (tblgen native build)

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>

.github/workflows/main.yml

@@ -34,9 +34,14 @@ jobs:
             cross_cmake_args: >-
               -DCMAKE_SYSTEM_NAME=Linux
               -DCMAKE_SYSTEM_PROCESSOR=riscv64
+              -DCMAKE_C_COMPILER=riscv64-linux-gnu-gcc
+              -DCMAKE_CXX_COMPILER=riscv64-linux-gnu-g++
               -DWASI_SDK_LLDB=OFF
             env:
               WASI_SDK_CI_SKIP_SYSROOT: 1
+              WASI_SDK_CI_TOOLCHAIN_LLVM_CMAKE_ARGS: >-
+                -DCMAKE_SYSTEM_NAME=Linux
+                -DCMAKE_SYSTEM_PROCESSOR=riscv64
 
           - artifact: arm64-macos
             os: macos-14

ci/docker/Dockerfile.riscv64-linux

@@ -16,13 +16,11 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
   ccache \
   && rm -rf /var/lib/apt/lists/*
 
-# Set the cross-compiler as the default so that cmake picks it up via CC/CXX
-# and sets CMAKE_CROSSCOMPILING=TRUE, which causes LLVM to build a native
-# tablegen before cross-compiling the rest of the toolchain.
-ENV CC=riscv64-linux-gnu-gcc
-ENV CXX=riscv64-linux-gnu-g++
-
 # Cargo needs an explicit linker when cross-compiling for riscv64.
+# The C/C++ cross-compiler is passed via CMAKE_C/CXX_COMPILER cmake flags
+# rather than CC/CXX env vars so that LLVM's native tblgen sub-build can
+# still find the host compiler (cmake cache vars are not inherited by
+# subprocess cmake invocations, but env vars are).
 ENV CARGO_TARGET_RISCV64GC_UNKNOWN_LINUX_GNU_LINKER=riscv64-linux-gnu-gcc
 
 # Tell programs to cache in a location that both isn't a `--volume` mounted root