with-astro example PostCSS exception at build

[mmccoy]

Building the example on code sandbox (and locally), I receive:

(node:57) UnhandledPromiseRejectionWarning: Error: Loading PostCSS Plugin failed: The argument 'filename' must be a file URL object, file URL string, or absolute path string. Received undefined

(@undefined)
    at load (/sandbox/node_modules/postcss-load-config/src/plugins.js:28:11)
    at /sandbox/node_modules/postcss-load-config/src/plugins.js:53:16
    at Array.map (<anonymous>)
    at plugins (/sandbox/node_modules/postcss-load-config/src/plugins.js:52:8)
    at AsyncFunction.transformAsync (/sandbox/node_modules/@snowpack/plugin-postcss/worker.js:20:17)
    at MessagePort.<anonymous> (/sandbox/node_modules/workerpool/src/worker.js:125:27)
    at MessagePort.[nodejs.internal.kHybridDispatch] (internal/event_target.js:399:24)
    at MessagePort.exports.emitMessage (internal/per_context/messageport.js:18:26)
(Use `node --trace-warnings ...` to show where the warning was created)
(node:57) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). To terminate the node process on unhandled promise rejection, use the CLI flag `--unhandled-rejections=strict` (see https://nodejs.org/api/cli.html#cli_unhandled_rejections_mode). (rejection id: 1)
(node:57) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.

[mmccoy]

Ooops, sorry, dupe of #248