[ci] Add SSH key before init-submodules for access to rocket-gpu

Need to do the same for every init-submodule script run

.github/actions/prepare-gpu/action.yml

@@ -20,6 +20,8 @@ runs:
     - name: Init submodules to ensure stimuli is located
       run: |
         conda activate ${{ env.conda-env-name-no-time }}-$(date --date "${{ env.workflow-timestamp }}" +%Y%m%d)-${{ inputs.toolchain }}
+        eval `ssh-agent -s`
+        ssh-add - <<< '${{ secrets.PRIVATE_SSH_KEY }}'
         ./scripts/init-submodules-no-riscv-tools.sh --force
       shell: bash -leo pipefail {0}