fix(UpdatePlayerGameMetricsAction): use firstOrCreate to prevent race condition duplicate inserts

[wescopeland]

Resolves:

Root Cause
Two concurrent UpdatePlayerGameMetricsJob instances for the same user+game can both query for an existing PlayerAchievementSet. Both find nothing, both try to INSERT, and the second one hits a unique constraint.

We're basically hand-rolling a first() + new + save() pattern. This PR replaces that pattern with a firstOrCreate() which should handle the operation atomically.

[Jamiras]

Suggest just fetching this here to avoid an extra lookup in a few lines:

if ($isUntouchedSubset) {
    $playerAchievementSet = PlayerAchievementSet...first();
    if (!$playerAcheivementSet) {
        continue;
    }
} else {
    $playerAchievementSet = PlayerAchievementSet::firstOrCreate(...);
}