Squoosh CLI incorrectly handles grayscale PNGs

[shuding]

Before you start
Please take a look at the FAQ as well as the already opened issues! If nothing fits your problem, go ahead and fill out the following template:

Describe the bug
Squoosh CLI doubles the image and changed the hue for grayscale PNGs:

It can be reproduced with both WebP and OxiPNG, but it works expectedly in the web app.

To Reproduce
Steps to reproduce the behavior:

1. Download https://imgur.com/wgqaUdn (or https://i.imgur.com/eTgdpzy.png)
2. Run squoosh-cli --webp auto image.png
3. See bug

Expected behavior
It should not change the image content.

Version:

• OS w/ version: macOS 11.2.2
• Browser w/ version: n/a
• Node version: v14.16.0
• npm version: 6.14.11

Is your issue related to the quality of image compression?
Please attach original and output images (you can drag & drop to attach).

• Original image

• Output image from Squoosh

Additional context, screenshots, screencasts

Here's a full screenshot of commands I ran:

[developit]

As per @RReverser: this is likely caused by the Rust-based PNG decoder, which we don't use in the browser version of Squoosh (matches up with the bug description). We are configuring a greyscale-to-RGB transform, but it doesn't seem to be functioning as the decoding code expects:

squoosh/codecs/png/src/lib.rs

Lines 45 to 46 in ef77da9

	// Transformations::EXPAND will make sure color_type is either
	// RGBA or RGB. If it’s RGB, we need inject an alpha channel.