ci: dump PATH/which-python before importing torch in test-kernels

The previous run installed torch into /usr/local/bin/python3.12 (uv said
"Using Python 3.12.13 environment at: /usr/local") but the next step's
`python -c "import torch"` raised ModuleNotFoundError. Add an env+which
dump before the import to identify which interpreter the runner exec env
actually resolves and where torch actually landed.

Also keep the GITHUB_PATH prepend of /usr/local/bin so subsequent steps
have python3.12 ahead of any other interpreter on PATH.

Signed-off-by: Matthias Gehre <matthias.gehre@amd.com>

Author: mgehre-amd

.github/workflows/build-rocm-wheels.yml

@@ -152,6 +152,9 @@ jobs:
       - name: Checkout code
         uses: actions/checkout@v4
 
+      - name: Ensure python3.12 wins over Debian's python3.11 on PATH
+        run: echo "/usr/local/bin" >> "$GITHUB_PATH"
+
       - name: Download wheel artifact
         uses: actions/download-artifact@v4
         with:
@@ -212,6 +215,18 @@ jobs:
 
       - name: Prepare test environment
         run: |
+          # Diagnose: which python does the runner exec env actually pick up?
+          echo "=== env (PATH-related) ==="
+          env | grep -E '^(PATH|VIRTUAL_ENV|PYTHONPATH|PYTHONHOME|RUNNER_TOOL_CACHE)=' || true
+          echo "=== which / where ==="
+          which python python3 python3.11 python3.12 2>&1 || true
+          ls -la /usr/local/bin/python* /usr/bin/python* 2>&1 || true
+          echo "=== python -V via 'python' and 'python3.12' ==="
+          python -V 2>&1 || true
+          python3.12 -V 2>&1 || true
+          echo "=== where is torch installed ==="
+          python3.12 -c "import torch, sys; print(torch.__file__); print(sys.path)" 2>&1 || true
+
           # Remove the source vllm/ directory so Python imports from the
           # installed wheel (which has compiled C extensions) instead of
           # the checkout source tree (which has no .so files).