Add environment variable for WebExtensions `example.json`

[caugner]

Currently, the URL is hard-coded:

rari/crates/rari-deps/src/web_ext_examples.rs

Lines 6 to 13 in ced5a33

	pub fn update_web_ext_examples(base_path: &Path) -> Result<(), DepsError> {
	get_json(
	"web_ext_examples",
	"https://raw.githubusercontent.com/mdn/webextensions-examples/main/examples.json",
	base_path,
	)?;
	Ok(())
	}

Blocks mdn/webextensions-examples#640.