flowy: set nixos as default EFI boot entry, always

hosts/by-name/flowy/default.nix

@@ -1,4 +1,4 @@
-{ ... }:
+{ lib, pkgs, ... }:
 {
   imports = [
     ./fs.nix
@@ -26,4 +26,32 @@
   # add an entry to boot into Windows, as if it had been launched directly from the BIOS.
   boot.loader.systemd-boot.rebootForBitlocker = true;
   boot.loader.systemd-boot.windows.primary.efiDeviceHandle = "HD0b";
+
+  system.activationScripts.makeDefaultBootEntry = {
+    text = let
+      makeDefaultBootEntry = pkgs.writeShellApplication {
+        name = "makeDefaultBootEntry";
+        runtimeInputs = with pkgs; [
+          efibootmgr
+          gnugrep
+        ];
+        text = ''
+          # configure the EFI firmware to boot into NixOS by default.
+          # do this by querying the active boot entry, and just making that be the default.
+          # this is needed on flowy because enabling secure boot / booting into Windows
+          # resets the default boot order; manually reconfiguring that is tiresome.
+          efi=$(efibootmgr)
+          bootCurrent=$(echo "$efi" | grep '^BootCurrent: ')
+          bootCurrent=''${bootCurrent/BootCurrent: /}
+          bootOrder=$(echo "$efi" | grep '^BootOrder: ')
+          bootOrder=''${bootOrder/BootOrder: /}
+          # prepend the current boot entry to the boot order...
+          newBootOrder="$bootCurrent,$bootOrder"
+          # but in a way that's idempotent :)
+          newBootOrder=''${newBootOrder/$bootCurrent,$bootCurrent/$bootCurrent}
+          (set -x; efibootmgr -o "$newBootOrder")
+        '';
+      };
+    in lib.getExe makeDefaultBootEntry;
+  };
 }