lib/installation: Add new FlatpakUpdateFlag to prevent pruning the repo

Add a new FLATPAK_UPDATE_FLAGS_NO_PRUNE so that we can specify from clients
when we don't want to perform a prune right after updating a flatpak, even
if no FLATPAK_UPDATE_FLAGS_NO_PULL flag has been set.

https://github.com/flatpak/flatpak/issues/1703

Closes: #1704
Approved by: alexlarsson
This commit is contained in:
Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f