Fix Flatpak Builder Linter Warnings

[RayJW]

Closed #466 accidentally. This is the same PR, which fixes a Flatpak builder linting issue by replacing a plain cmake build with cmake-ninja.

[flathubbot]

Started test build 169569

[flathubbot]

Build 169569 successful
To test this build, install it from the testing repository:

flatpak install --user https://dl.flathub.org/build-repo/152669/net.lutris.Lutris.flatpakref

[flathubbot]

Started test build 182764

[flathubbot]

Build 182764 successful
To test this build, install it from the testing repository:

flatpak install --user https://dl.flathub.org/build-repo/165963/net.lutris.Lutris.flatpakref

[flathubbot]

Started test build 188504

[flathubbot]

Build 188504 successful
To test this build, install it from the testing repository:

flatpak install --user https://dl.flathub.org/build-repo/171775/net.lutris.Lutris.flatpakref