Use flathub-infra/flatpak-github-actions/ to build weekly flatpak

Fixes getting-things-gnome#1184

Our current `flatpak/flatpak-github-actions` is not longer working,
because it is using a deprecated version of the `upload-artifact` GHA.
From
https://github.blog/changelog/2024-04-16-deprecation-notice-v3-of-the-artifact-actions/:

> Starting January 30th, 2025, GitHub Actions customers will no longer
> be able to use v3 of https://github.com/actions/upload-artifact or
> https://github.com/actions/download-artifact.

We're using upload-artifact from flatpak-github-actions, and the issue
has been filed there as
flatpak/flatpak-github-actions#214.
flatpak/flatpak-github-actions#184 is expected
to fix that, but the associated PR
flatpak/flatpak-github-actions#198 hasn't been
merged.

The GHA has been forked at
https://github.com/flathub-infra/flatpak-github-actions, and the issue
was fixed there back in July:
flathub-infra/flatpak-github-actions@efb7f71

So let's switch to that. I don't like that it's not versioned and we
need to use the master branch, which may add breaking changes at any
time. But it's better than our current solution, which can no longer
work.

Tested on a fork, and the artifact is created correctly.

.github/workflows/weekly_flatpak.yml

@@ -16,7 +16,7 @@ jobs:
       options: --privileged
     steps:
     - uses: actions/checkout@v4
-    - uses: flatpak/flatpak-github-actions/flatpak-builder@v6
+    - uses: flathub-infra/flatpak-github-actions/flatpak-builder@master
       with:
         bundle: org.gnome.GTG.Devel.flatpak
         manifest-path: build-aux/org.gnome.GTG.Devel.json