Update to use PowerShell on all OSes

[mheap]

$content = echo '{"requires": {"phpunit/phpunit": "^9"}}' | ConvertFrom-json
$content.requires.{phpunit/phpunit} = "^123"
$content | ConvertTo-json

Available on all runners. See https://github.com/actions/virtual-environments/blob/master/images/linux/Ubuntu1804-README.md

[mheap]

See 5c2511a