pythonInterpreters.pypy310_prebuilt: set mainProgram

Without the change the eval fails as:

    $ nix build --no-link -f. pythonInterpreters.pypy310_prebuilt.tests.test-packageOverrides
    error:
       … in the condition of the assert statement
         at pkgs/development/interpreters/python/tests.nix:212:11:
          211|           in
          212|           assert myPython.pkgs.foobar == myPython.pkgs.numpy;
             |           ^
          213|           myPython.withPackages (ps: with ps; [ foobar ]);

       … while evaluating the attribute 'outPath'
         at lib/customisation.nix:421:7:
          420|         drv.drvPath;
          421|       outPath =
             |       ^
          422|         assert condition;

       (stack trace truncated; use '--show-trace' to show the full, detailed trace)

       error: attribute 'mainProgram' missing
       at pkgs/by-name/me/meson/package.nix:75:52:
           74|           --replace-fail "PythonExternalProgram('python3', mesonlib.python_command)" \
           75|                          "PythonExternalProgram('${python3.meta.mainProgram}', mesonlib.python_command)"
             |                                                    ^
           76|         substituteInPlace mesonbuild/modules/python3.py \

(cherry picked from commit dc251ab95c)

pkgs/development/interpreters/python/pypy/prebuilt.nix

@@ -176,6 +176,7 @@ stdenv.mkDerivation {
   meta = with lib; {
     homepage = "http://pypy.org/";
     description = "Fast, compliant alternative implementation of the Python language (${pythonVersion})";
+    mainProgram = "pypy";
     license = licenses.mit;
     platforms = lib.mapAttrsToList (arch: _: arch) downloadUrls;
   };