[Fix test] Fix ensurepip missing (#35437)

Passed manual runs:
* [grpc/core/master/linux/grpc_interop_tocloud](https://fusion2.corp.google.com/invocations/70b9a382-03b0-46ce-9181-cd3382ae9ee7) (Ruby failed)
* [grpc/core/master/linux/arm64/grpc_basictests_python](https://fusion2.corp.google.com/invocations/279d959a-3d3f-4d0c-a0a3-712fefca14fe/targets)
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Closes #35437

COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/35437 from XuanWang-Amos:fix_ensurepip_missing 8c572dcfa5
PiperOrigin-RevId: 595520790

tools/run_tests/helper_scripts/build_python.sh

@@ -124,9 +124,9 @@ if [[ "$(inside_venv)" ]]; then
 else
   # Instantiate the virtualenv from the Python version passed in.
   $PYTHON -m pip install --user virtualenv==20.25.0
-  # Use --no-seed to prevent virtualenv from installing seed packages.
-  # Otherwise it might not find cython module while building grpcio.
-  $PYTHON -m virtualenv --no-seed "$VENV"
+  # Skip wheel and setuptools and manually install later. Otherwise we might
+  # not find cython module while building grpcio.
+  $PYTHON -m virtualenv --no-wheel --no-setuptools "$VENV"
   VENV_PYTHON="$(pwd)/$VENV/$VENV_RELATIVE_PYTHON"
 fi
 
@@ -134,7 +134,7 @@ pip_install() {
   $VENV_PYTHON -m pip install "$@"
 }
 
-$VENV_PYTHON -m ensurepip --upgrade
+pip_install --upgrade pip
 pip_install --upgrade wheel
 pip_install --upgrade setuptools==66.1.0