nixos/systemd-boot: Fix regression in builder script

Avoid running Python scripts in the root of the package, as this
triggers `os.listdir` on the Nix store directory during import. This
operation can be time-consuming on large store directories
(see issue #283795 for more details).

The issue was initially fixed in #284153 but was reverted in #306339.

Co-authored-by: Sönke Hahn <soenkehahn@gmail.com>

nixos/modules/system/boot/loader/systemd-boot/systemd-boot.nix

@@ -24,6 +24,8 @@ let
   systemdBootBuilder = pkgs.substituteAll rec {
     name = "systemd-boot";
 
+    dir = "bin";
+
     src = checkedSource;
 
     isExecutable = true;
@@ -86,7 +88,7 @@ let
 
   finalSystemdBootBuilder = pkgs.writeScript "install-systemd-boot.sh" ''
     #!${pkgs.runtimeShell}
-    ${systemdBootBuilder} "$@"
+    ${systemdBootBuilder}/bin/systemd-boot "$@"
     ${cfg.extraInstallCommands}
   '';
 in {