fix(flatpak): use rustup to install latest Rust in SDK 23.08

SDK 24.08 doesn't have javascriptcoregtk-4.1 (uses webkit 6.0).
SDK 23.08 has Rust 1.81 which doesn't support edition2024.
Solution: use SDK 23.08 with webkit2gtk 4.1 but install Rust via rustup.

.github/workflows/release-flatpak.yml

@@ -77,10 +77,9 @@ jobs:
       - name: Install runtimes
         run: |
           flatpak --user install -y --noninteractive flathub \
-            org.freedesktop.Platform//24.08 \
-            org.freedesktop.Sdk//24.08 \
-            org.freedesktop.Sdk.Extension.node22//24.08 \
-            org.freedesktop.Sdk.Extension.rust-stable//24.08
+            org.freedesktop.Platform//23.08 \
+            org.freedesktop.Sdk//23.08 \
+            org.freedesktop.Sdk.Extension.node20//23.08
 
       - name: Build Flatpak
         env: