az image builder create claims invalid JSON

**Describe the bug**
when running az image builder create, when passing a template JSON, the CLI always claims that the JSON is invalid.

**To Reproduce**
az image builder create -g $imageResourceGroup -n $TemplateName --image-template ImageTemplate.json

Same for any JSON - you can for example use this one: https://raw.githubusercontent.com/azure/azvmimagebuilder/master/quickquickstarts/0_Creating_a_Custom_Windows_Managed_Image/helloImageTemplateWin.json

(setting subscription etc. to valid values)

I tried with 10 different files - all same result and all are valid JSON.

**Expected behavior**
This should lead to the same resource creation as this:

az resource create --resource-group $imageResourceGroup --properties "@./ImageTemplate.json" --is-full-object `
--resource-type Microsoft.VirtualMachineImages/imageTemplates -n $TemplateName

the az resource command works fine - passing in identical JSON.

**Environment summary**
az cli 2.25.0 on Windows 10