add qemu patch for unshare

modules/desktop/binfmt.nix

@@ -1 +0,0 @@
-{boot.binfmt.emulatedSystems = ["aarch64-linux"];}

modules/desktop/binfmt/default.nix

@@ -0,0 +1,11 @@
+{
+  boot.binfmt.emulatedSystems = ["aarch64-linux"];
+  nixpkgs.overlays = [
+    (_: super: {
+      # Because of https://github.com/NixOS/nixpkgs/pull/378579
+      qemu = super.qemu.overrideAttrs (old: {
+        patches = (old.patches or []) ++ [./qemu.patch];
+      });
+    })
+  ];
+}

modules/desktop/binfmt/qemu.patch

@@ -0,0 +1,16 @@
+diff --git a/util/rcu.c b/util/rcu.c
+index fa32c942e4..8ae899f74e 100644
+--- a/util/rcu.c
++++ b/util/rcu.c
+@@ -409,12 +409,6 @@ static void rcu_init_complete(void)
+ 
+     qemu_event_init(&rcu_call_ready_event, false);
+ 
+-    /* The caller is assumed to have BQL, so the call_rcu thread
+-     * must have been quiescent even after forking, just recreate it.
+-     */
+-    qemu_thread_create(&thread, "call_rcu", call_rcu_thread,
+-                       NULL, QEMU_THREAD_DETACHED);
+-
+     rcu_register_thread();
+ }