ci: install gcovr via apt

Change-Id: I017ec6e4281a0348eca0361643cb3af0167c420f
Refs: #4771
diff --git a/.jenkins.d/30-coverage.sh b/.jenkins.d/30-coverage.sh
index 92f2bb8..46d3cf0 100755
--- a/.jenkins.d/30-coverage.sh
+++ b/.jenkins.d/30-coverage.sh
@@ -9,7 +9,7 @@
 if [[ $JOB_NAME == *"code-coverage" ]]; then
     gcovr --object-directory=build \
           --output=build/coverage.xml \
-          --filter="$PWD/ndn-cxx" \
+          --exclude="$PWD/(examples|tests)" \
           --root=. \
           --xml