cli: automatically use distro qualities if available (#166961)

A fully-functioning CLI with tunnel capabilities requires product-specific
configuration that is not included in the OSS. This change has the CLI's build
script automatically look for a peer `vscode-distro` folder and, in a debug
build, set its build variables based on that.

It also works to set correct variables if vscode-distro is not found, based on
the OSS product.json (though this is not sufficient to run a fully fledged
tunnel.)