main/postmarketos-ui-phosh: no gnome-software first run dialog

Disable the first run screen in GNOME software, which invites the user to browse and install apps. We do not want people to use GNOME software to do that when they open it for the first time in postmarketOS after installing, we want them to do updates. So this dialog is misleading for our users, and will lead to a bad experience for those who follow what it suggests.

I think there's more wrong with that dialog (in context of postmarketOS), more detailed reasoning and screenshot in: !1449 (comment 385456592)

I've verified that this works by cherry-picking to v20.05, building an image and booting it on the phinephone. The first run dialog in GNOME software is gone.