Set SUID bit on chrome-sandbox for Debian

This tweaks Linux packages for Riot to SUID `chrome-sandbox` after install. This
is required as of Electron 5 for certain distros, such as Debian.

This change has also been provided to `electron-builder` upstream, so ideally
they'll include it in the future and this becomes redundant.

Fixes https://github.com/vector-im/riot-web/issues/10509

package.json

@@ -175,6 +175,9 @@
         "StartupWMClass": "riot"
       }
     },
+    "deb": {
+      "afterInstall": "electron_app/build/linux/after-install.tpl"
+    },
     "mac": {
       "category": "public.app-category.social-networking"
     },