proxmox-backup-client: move RUSTFLAGS env variables into env for structuredAttrs

pkgs/by-name/pr/proxmox-backup-client/package.nix

@@ -124,13 +124,14 @@ rustPlatform.buildRustPackage {
     "--bin=pxar"
   ];
 
-  RUSTFLAGS = [ "-L.dep-stubs" ];
+  env = {
+    RUSTFLAGS = toString [ "-L.dep-stubs" ];
+    # pbs-buildcfg requires this set, would be the git commit id
+    REPOID = "";
+  };
 
   doCheck = false;
 
-  # pbs-buildcfg requires this set, would be the git commit id
-  REPOID = "";
-
   nativeBuildInputs = [
     pkgconf
     rustPlatform.bindgenHook