stm32: do not use callPackage on something that isn't a derivation

stm32.nix

@@ -1,7 +1,7 @@
 { pkgs ? import <nixpkgs> {}, rustManifest ? ./stm32/channel-rust-nightly.toml }:
 
 let
-  jobs = pkgs.callPackage ./stm32/default.nix {
+  jobs = import ./stm32/default.nix {
     inherit rustManifest;
     mozillaOverlay = import <mozillaOverlay>;
   };