Adjust the npm install command
Adjust the npm install command

The unit tests image installs `npm` via `nvm`. This means that we need
to source the shell script to point to the right location for `npm`
command.

The `--no-audit` flag is used to suppress any warnings when building the
image on concourse, maintaining the current approach.

alphagov/notifications-admin,alphagov/notifications-admin,alphagov/notifications-admin,alphagov/notifications-admin