fix(ci): source IDF export before NVS matrix generation in QEMU tests

The generate_nvs_matrix.py script needs the IDF venv's python
(which has esp_idf_nvs_partition_gen installed) rather than the
system /usr/bin/python3 which doesn't have the package.

Co-Authored-By: claude-flow <ruv@ruv.net>

.github/workflows/firmware-qemu.yml

@@ -134,6 +134,7 @@ jobs:
 
       - name: Generate NVS matrix
         run: |
+          . $IDF_PATH/export.sh
           python3 scripts/generate_nvs_matrix.py \
             --output-dir firmware/esp32-csi-node/build/nvs_matrix \
             --only ${{ matrix.nvs_config }}