postgresqlJitPackages.lantern: fix build

The link_llvm_objects.sh script used a hardcoded /bin/bash shebang,
which failed during a JIT-enabled build.

pkgs/servers/sql/postgresql/ext/lantern.nix

@@ -19,6 +19,10 @@ stdenv.mkDerivation (finalAttrs: {
     fetchSubmodules = true;
   };
 
+  postPatch = ''
+    patchShebangs --build scripts/link_llvm_objects.sh
+   '';
+
   nativeBuildInputs = [
     cmake
   ];