CI Coverity: fix build abortion

see https://github.com/MartinPulec/UltraGrid/actions/runs/3658131620/jobs/6182827419

For Vulkan, the memory exhaustion perhaps occurs, thus use only one
concurrent process to build.

.github/workflows/coverity-scan.yml

@@ -49,7 +49,7 @@ jobs:
     - name: Build with cov-build
       if: ${{ env.coverity_token }}
       run: |
-        /tmp/cov-analysis/bin/cov-build --dir cov-int make -j4
+        /tmp/cov-analysis/bin/cov-build --dir cov-int make
     - name: Submit the result to Coverity Scan
       if: ${{ env.coverity_token }}
       run: |