ci: Build flatpak-builder against latest baseapp

(cherry picked from commit 2338049)

.github/workflows/ci.yml

@@ -32,6 +32,8 @@ jobs:
     - name: Build org.flatpak.Builder.BaseApp
       run: |
         jq '."base-version" = "localtest"' org.flatpak.Builder.BaseApp.json >> org.flatpak.Builder.BaseApp-modified.json
+        runtime_version="$(jq '."runtime-version"' org.flatpak.Builder.BaseApp.json)"
+        echo "BASEAPP_RUNTIME_VERSION=${runtime_version}" >> $GITHUB_ENV
         git submodule update --init --recursive
         flatpak-builder --verbose --user --sandbox --force-clean --repo=repo \
           --install-deps-from=flathub --default-branch=localtest --ccache \
@@ -42,9 +44,10 @@ jobs:
         git clone --depth=1 --branch master --recursive --single-branch https://github.com/flathub/org.flatpak.Builder.git
         git submodule update --init --recursive
         cd org.flatpak.Builder
+        jq '."runtime-version" = $ENV.BASEAPP_RUNTIME_VERSION | ."base-version" = "localtest"' org.flatpak.Builder.json >> org.flatpak.Builder-modified.json
         flatpak-builder --verbose --user --sandbox --force-clean --repo=repo \
           --install-deps-from=flathub --default-branch=localtest --ccache \
-          --install builddir org.flatpak.Builder.json
+          --install builddir org.flatpak.Builder-modified.json
 
     - name: Check if org.flatpak.Builder launches
       run: |