Make Jenkins Node tests always build from source

tools/jenkins/grpc_interop_node/build_interop.sh

@@ -45,4 +45,4 @@ make install-certs
 
 # build Node interop client & server
 npm install -g node-gyp
-(npm install && node-gyp rebuild)
+npm install --unsafe-perm --build-from-source

tools/run_tests/build_node.sh

@@ -36,4 +36,4 @@ CONFIG=${CONFIG:-opt}
 # change to grpc repo root
 cd $(dirname $0)/../..
 
-npm install --unsafe-perm
+npm install --unsafe-perm --build-from-source