fix(ci): add elfutils for Flatpak build

The libayatana modules added in 2923627 require eu-strip to strip
debug symbols during flatpak-builder execution.

Author: farion1231

.github/workflows/release.yml

@@ -56,7 +56,8 @@ jobs:
             libssl-dev \
             rpm \
             flatpak \
-            flatpak-builder
+            flatpak-builder \
+            elfutils
           # GTK/GLib stack for gdk-3.0, glib-2.0, gio-2.0
           sudo apt-get install -y --no-install-recommends \
             libgtk-3-dev \