CI: Use manifest hash as Flatpak cache key

https://github.com/bilelmoussaoui/flatpak-github-actions/issues/80
This issue is preventing the removal of the cache-key in the actions
inputs to use the default cache key.

.github/workflows/flatpak.yml

@@ -46,7 +46,7 @@ jobs:
         with:
           bundle: obs-studio-${{ steps.setup.outputs.commitHash }}.flatpak
           manifest-path: CI/flatpak/com.obsproject.Studio.json
-          cache-key: flatpak-builder-${{ github.sha }}
+          cache-key: flatpak-builder-${{ hashFiles('CI/flatpak/com.obsproject.Studio.json') }}
           mirror-screenshots-url: https://dl.flathub.org/repo/screenshots
           branch: ${{ matrix.branch }}
 

.github/workflows/main.yml

@@ -399,7 +399,7 @@ jobs:
         with:
           bundle: obs-studio-flatpak-${{ env.OBS_GIT_HASH }}.flatpak
           manifest-path: CI/flatpak/com.obsproject.Studio.json
-          cache-key: flatpak-builder-${{ github.sha }}
+          cache-key: flatpak-builder-${{ hashFiles('CI/flatpak/com.obsproject.Studio.json') }}
 
   windows_package:
     name: '03 - Windows Installer'