Use Lutris DB platform value over spec

It is unlikely that this would ever change for a game so it's less important to ensure that we're prioritising the correct source, but while we could likely try to infer the platform from what fields are used in the spec and what form they're in, the database contains an explicit value which is more likely to be used when deciding how to launch a game than an inference process.

We also never set `platform` in `scan_spec` so this is also a minor optimisation.

src/scan/launchers/lutris.rs

@@ -74,7 +74,7 @@ impl Pending {
                     // Lutris updates the spec, but not the database,
                     // so we prefer the spec version.
                     prefix: spec.prefix.or(db.prefix),
-                    platform: spec.platform.or(db.platform),
+                    platform: db.platform.or(spec.platform),
                     install_dir: spec.install_dir.or(db.install_dir),
                 }
             }