build: switch to python3

Refs: #5095
Change-Id: I7e7a3a5cd49d69c6e389060ccf8fe30599ebd876
diff --git a/.jenkins.d/01-ndn-cxx.sh b/.jenkins.d/01-ndn-cxx.sh
index bec70d4..0ae5022 100755
--- a/.jenkins.d/01-ndn-cxx.sh
+++ b/.jenkins.d/01-ndn-cxx.sh
@@ -43,7 +43,7 @@
 pushd ndn-cxx >/dev/null
 
 ./waf --color=yes configure --disable-static --enable-shared --without-osx-keychain
-./waf --color=yes build -j${WAF_JOBS:-1}
+./waf --color=yes build -j$WAF_JOBS
 sudo_preserve_env PATH -- ./waf --color=yes install
 
 popd >/dev/null