CI/CD: Add the ability to skip packaging specific driver versions

Currently, we unconditionally package every driver version whenever a commit is pushed to `master`, which is highly wasteful — it triggers over 100 GB of downloads from NVIDIA’s servers.

A couple of ideas to avoid this:

\- **Skip existing versions**: Before packaging, check whether the driver version is already available on Flathub; if it is, skip it.
\- **Control via commit message**: Use keywords in the commit message to specify which versions to package.

**Note:** We must still retain the ability to repackage any version if needed — for example, when we discover that important files were omitted (see commits https://github.com/flathub/org.freedesktop.Platform.GL.nvidia/commit/748f5f3da10a396a7e0a72b55ff0fe28c715f581 and https://github.com/flathub/org.freedesktop.Platform.GL.nvidia/commit/a31710791897b8219763dbfb4f8c4088f6080277 as examples).