python3{11,12}Packages.pdm-backend: set Git user.name for tests

At least one Hydra Darwin builder has an empty real name field for
at least one Nix build user, which causes Git to infer an empty
`user.name` and break.

pkgs/development/python-modules/pdm-backend/default.nix

@@ -46,6 +46,7 @@ buildPythonPackage rec {
 
     # tests require a configured git identity
     export HOME=$TMPDIR
+    git config --global user.name nixbld
     git config --global user.email nixbld@localhost
   '';