karma: set CHROMIUM_BIN, not CHROME_BIN

karma is looking for the chromium exedcutable, but the playwright one is still
called "chrome". I didn't noticed this because it would fall back to the debian install
chromium instance then.

karma.common.conf.js

@@ -2,7 +2,7 @@ module.exports = async function(config) {
   const { installBrowsersForNpmInstall, registry } = require('playwright-core/lib/server');
   await installBrowsersForNpmInstall(["firefox", "chromium"]);
   process.env.FIREFOX_BIN = registry.findExecutable("firefox").executablePath();
-  process.env.CHROME_BIN = registry.findExecutable("chromium").executablePath();
+  process.env.CHROMIUM_BIN = registry.findExecutable("chromium").executablePath();
 
   config.set({
     basePath: 'dist',