workflows: Use flatpak-builder action master version

Since "flatpak-builder" action uses deprecated "upload-artifact"
version, that can stop working soon, disable it and upload ourselves.
This is supported in master, but not in any tagged version,
so for now switch to using action from master branch.

Also bump used runtime to GNOME 47 when building.

.github/workflows/flatpak-nightly.yml

@@ -33,7 +33,7 @@ jobs:
         flatpak --system install -y --noninteractive flathub org.freedesktop.Sdk.Extension.rust-stable/${{ matrix.arch }}/24.08
         flatpak --system install -y --noninteractive flathub org.freedesktop.Sdk.Extension.llvm18/${{ matrix.arch }}/24.08
     - name: Build
-      uses: bilelmoussaoui/flatpak-github-actions/flatpak-builder@v6
+      uses: flatpak/flatpak-github-actions/flatpak-builder@master
       with:
         bundle: com.github.rafostar.Clapper.flatpak
         manifest-path: pkgs/flatpak/com.github.rafostar.Clapper-nightly.json
@@ -41,3 +41,10 @@ jobs:
         repository-url: https://nightly.gnome.org/gnome-nightly.flatpakrepo
         cache-key: flatpak-builder-${{ github.sha }}-testing-${{ github.run_number }}
         arch: ${{ matrix.arch }}
+        upload-artifact: false
+    - name: Upload
+      uses: actions/upload-artifact@v4
+      with:
+        name: clapper-flatpak-nightly-${{ matrix.arch }}
+        path: com.github.rafostar.Clapper.flatpak
+        if-no-files-found: error