ci: require gcovr 5.1 for code coverage reports

Stop passing `-j`, it wasn't very useful and it's broken in gcovr 5.1

Change-Id: I265b23eb21a5c229b8ce7b4099b342cecd4925a3
diff --git a/.jenkins.d/30-coverage.sh b/.jenkins.d/30-coverage.sh
index 2908b17..4ff1c3e 100755
--- a/.jenkins.d/30-coverage.sh
+++ b/.jenkins.d/30-coverage.sh
@@ -4,14 +4,13 @@
 if [[ $JOB_NAME == *"code-coverage" ]]; then
     # Generate an XML report (Cobertura format) and a detailed HTML report using gcovr
     # Note: trailing slashes are important in the paths below. Do not remove them!
-    gcovr -j$WAF_JOBS \
-          --object-directory build \
+    gcovr --object-directory build \
           --filter src/ \
           --exclude-throw-branches \
           --exclude-unreachable-branches \
-          --print-summary \
+          --cobertura build/coverage.xml \
           --html-details build/gcovr/ \
-          --xml build/coverage.xml
+          --print-summary
 
     # Generate a detailed HTML report using lcov
     lcov --quiet \