build: archive test result for later analysis (#31146)

.github/workflows/nightly-builds.yml

@@ -167,7 +167,14 @@ jobs:
         with:
           report_paths: '**/target/test-reports/TEST-*.xml'
           fail_if_no_tests: false
-          skip_publishing: true
+
+      # Archive test results so we can do some diagnostics later
+      - name: Upload test results
+        uses: actions/upload-artifact@v2
+        if: success() || failure()        # run this step even if previous step failed
+        with:
+          name: test-results-${{ matrix.jdkVersion }}-${{ matrix.scalaVersion }}
+          path: '**/target/test-reports/TEST-*.xml'
 
       - name: Docs
         # Docs generation requires JDK 11. Checks with `startsWith` helps