linuxPackages.virtualbox: enable module signing during install

The bundled Makefile-footer.gmk passes
"CONFIG_MODULE_SIG= CONFIG_MODULE_SIG_ALL=" to the kernel's modules_install, so
the modules were installed unsigned and without the per-module certificate that
the kmod aggregator expects next to every external module since PR #571181.

Set VBOX_ALLOW_MODULE_SIGNING=1 and propagate kernelModuleMakeFlags (which
routes the signing rule to the deterministic kmodigest), so modinst
signs the modules and emits vboxdrv.ko.cer and friends alongside them.

Assisted-by: Kilo Code (GLM-5.3-Flash)

pkgs/os-specific/linux/virtualbox/default.nix

@@ -2,6 +2,7 @@
   stdenv,
   virtualbox,
   kernel,
+  kernelModuleMakeFlags,
 }:
 
 stdenv.mkDerivation {
@@ -18,7 +19,10 @@ stdenv.mkDerivation {
 
   env.KERN_DIR = "${kernel.dev}/lib/modules/${kernel.modDirVersion}/build";
 
-  makeFlags = [ "INSTALL_MOD_PATH=$(out)" ];
+  makeFlags = kernelModuleMakeFlags ++ [
+    "INSTALL_MOD_PATH=$(out)"
+    "VBOX_ALLOW_MODULE_SIGNING=1"
+  ];
   installTargets = [ "install" ];
 
   enableParallelBuilding = true;