build: switch to python3

Refs: #5095
Change-Id: I7e7a3a5cd49d69c6e389060ccf8fe30599ebd876
diff --git a/.jenkins b/.jenkins
index 78e1ca1..25bf743 100755
--- a/.jenkins
+++ b/.jenkins
@@ -1,9 +1,11 @@
 #!/usr/bin/env bash
 set -e
 
+export WAF_JOBS=${WAF_JOBS:-1}
+
 nanos() {
     # Cannot use date(1) because macOS does not support %N format specifier
-    python -c 'import time; print(int(time.time() * 1e9))'
+    python3 -c 'import time; print(int(time.time() * 1e9))'
 }
 
 for file in .jenkins.d/*; do