ci: use newer qemu-user-static for RVV cross tests

docker/setup-qemu-action@v3.0.0 pulls tonistiigi/binfmt:latest, which
pins an older qemu (~6.x/7.x) whose RVV implementation miscompiles the
vmulh / vmulhu variants emitted by gcc-14 at vlen=128: the test binary
runs forever under qemu while making no forward progress, hitting the
6h GHA job timeout. Local repro with qemu 11 / Ubuntu 24.04's
qemu-user-static (8.2.x) passes the same binary in ~30s.

Switch to the apt-shipped qemu-user-static so the cross-rvv job picks
up a maintained qemu without depending on the pinned docker image.

Author: DiamonDinoia

.github/workflows/cross-rvv.yml

@@ -35,9 +35,14 @@ jobs:
         sudo ln -srf $(which clang++-${{ matrix.sys.version }}) /usr/bin/clang++
         rm llvm.sh
     - name: Setup QEMU
-      uses: docker/setup-qemu-action@v3.0.0
-      with:
-        platforms: riscv64
+      # Use the qemu-user-static package shipped by Ubuntu 24.04 (qemu 8.2+)
+      # rather than docker/setup-qemu-action, which pins an older qemu via
+      # tonistiigi/binfmt and miscompiles RVV vmulh* at vlen=128 (the test
+      # binary hangs under qemu, with no forward progress, until the GHA
+      # 6h timeout).
+      run: |
+        sudo apt-get -y -qq update
+        sudo apt-get -y -qq --no-install-suggests --no-install-recommends install qemu-user-static
     - name: Setup Ninja
       run: |
         sudo apt-get -y -qq install ninja-build