feat: impl XL_PLATFORM for Linux

XL_PLATFORM for windows was implemented via a seperate PR at goatcorp/FFXIVQuickLauncher#1648. XoM uses their own fork of XLCommon that isn't the one in this repository, so it's fine to assume its only Linux here I think, I don't know anyone using this on a Mac

Author: Blooym

src/XIVLauncher.Common.Unix/UnixGameRunner.cs

@@ -1,6 +1,7 @@
 using System.Collections.Generic;
 using System.Diagnostics;
 using System.IO;
+
 using XIVLauncher.Common.Dalamud;
 using XIVLauncher.Common.PlatformAbstractions;
 using XIVLauncher.Common.Unix.Compatibility;
@@ -24,6 +25,7 @@ public UnixGameRunner(CompatibilityTools compatibility, DalamudLauncher dalamudL
     {
         if (dalamudOk)
         {
+            environment.Add("XL_PLATFORM", "Linux");
             return this.dalamudLauncher.Run(new FileInfo(path), arguments, environment);
         }
         else