Change to the executor workdir during npm upload

When using npm to install locally, it wants to write a node_modules
tree in the current path. Since the default path of a command is the
path where the playbook resides and this is not writeable on a zuul
executor, change to the writeable workdir first.

Change-Id: Ib4f632c0b9e3f5842e3cd649b6ca7ecb8042e7e3

roles/upload-npm/tasks/main.yaml

@@ -13,6 +13,8 @@
     version: v0.33.5
 
 - name: Upload tarball to npm
+  args:
+    chdir: "{{ zuul.executor.work_root }}"
   shell:
     executable: /bin/bash
     cmd: |