flatpak: detach process tracking thread after spawn

This makes sure the underlying thread implementation know to free
resources the moment the thread is no longer necessary, preventing leaks
from not manually collecting the thread.

src/os/flatpak.zig

@@ -112,6 +112,8 @@ pub const FlatpakHostCommand = struct {
     pub fn spawn(self: *FlatpakHostCommand, alloc: Allocator) !u32 {
         const thread = try std.Thread.spawn(.{}, threadMain, .{ self, alloc });
         thread.setName("flatpak-host-command") catch {};
+        // We don't track this thread, it will terminate on its own on command exit
+        thread.detach();
 
         // Wait for the process to start or error.
         self.state_mutex.lock();