Avoid flatpak install on applications alrady present in nix-flapatk old state

[gmodena]

Follow up from a conversation in #106.

nix-flaptak runs a flatpak install command during each activation. However, unless the user configures onActivation or auto, the operation is idempotent: flatpak detects that the app is already installed and skips reinstallation.

That said, this introduces unnecessary network operations (e.g., looking up the ref in its remote), which are both wasteful and can break network-less activations.

We should avoid running flatpak install <appId> is appId was already present in the previous nix-flatpak activation state.

[gmodena]

This should help mitigate #45 and general issues with network flakyness.