chromium: Attempt building on aarch64

This will probably go over the 10 hour limit, but we'll see.

pkgs/applications/networking/browsers/chromium/browser.nix

···
     maintainers = with maintainers; [ chaoflow bendlas ];
     license = licenses.bsd3;
     platforms = platforms.linux;
-    hydraPlatforms = if channel == "stable" then ["x86_64-linux"] else [];
+    hydraPlatforms = if channel == "stable" then ["aarch64-linux" "x86_64-linux"] else [];
   };
 })