Fix nyc and npm-update bugs due to unwritable HOME

Known to be affected:

• Jobs using npm-test:0.6.0, which inherits npm < ci-jessie.
• Jobs that will use node10-test, which inherits node10 < ci-stretch.

Fix by setting XDG_CONFIG_HOME and SPAWN_WRAP_SHIM_ROOT to /tmp.

Bug: T212602
Bug: T213014
Change-Id: I46eb8e1bd5ce4c00eb9125af07380d841c69a13c