Fix variable in workflow file

Author: ben221199

.github/workflows/flatpak-beta.yml

@@ -6,10 +6,10 @@ on:
 
 jobs:
   build:
-    runs-on: {{ matrix.os }}
     strategy:
       matrix:
         os: [ubuntu-24.04, ubuntu-24.04-arm]
+    runs-on: ${{ matrix.os }}
     steps:
       - uses: actions/checkout@v4
       - name: Install Flatpak