treewide: use runtimeShell in nixos/

This is needed for cross-compilation.

nixos/tests/virtualbox.nix

@@ -23,7 +23,7 @@ let
     guestAdditions = pkgs.linuxPackages.virtualboxGuestAdditions;
 
     miniInit = ''
-      #!${pkgs.stdenv.shell} -xe
+      #!${pkgs.runtimeShell} -xe
       export PATH="${lib.makeBinPath [ pkgs.coreutils pkgs.utillinux ]}"
 
       mkdir -p /run/dbus
@@ -80,7 +80,7 @@ let
       touch /mnt-root/boot-done
       hostname "${vmName}"
       mkdir -p /nix/store
-      unshare -m ${escapeShellArg pkgs.stdenv.shell} -c '
+      unshare -m ${escapeShellArg pkgs.runtimeShell} -c '
         mount -t vboxsf nixstore /nix/store
         exec "$stage2Init"
       '