Azure Devops pipeline with private artifact feed

Building static web apps from devops is looking good and easy to use. However we have run into a problem where we have some npm packages from our private feed, this is defined in our .npmrc file, however it appears to have problems installing packages.

.npmrc
```
registry=https://pkgs.dev.azure.com/xxx/_packaging/yyy/npm/registry/
always-auth=true;
```

For a normal pipeline build (using npm) we can define the feed in npm parameters:
```
- task: Npm@1
name: NpmInstall
inputs:
command: 'install'
workingDir: 'Clients/React'
customRegistry: 'useFeed'
customFeed: 'xxx'
```
Is it possible to specify a customFeed in the same way using the SPA pipeline task?

The pipeline output is:
```
Running 'npm install --unsafe-perm --production'...

npm ERR! code E401
npm ERR! Unable to authenticate, need: Bearer authorization_uri=https://login.windows.net/xxxx, Basic realm="https://pkgsprodcus1.pkgs.visualstudio.com/", TFS-Federated

npm ERR! A complete log of this run can be found in:
npm ERR! /root/.npm/_logs/2021-03-24T18_55_50_798Z-debug.log

```