usd-exchange: Upgrade pip in the test venv before installing the wheel

python -m venv seeds pip from the interpreter's ensurepip bundle, which is
23.0.1 for cp310 and 24.0 for cp311 in the manylinux image. Both vendor
packaging 21.3, whose manylinux arch list has no riscv64, so pip rejected
the manylinux_2_39_riscv64 wheel as unsupported on those two legs only.

Author: luhenry

.github/workflows/build-usd-exchange.yml

@@ -292,6 +292,7 @@ jobs:
 
           VENV=/tmp/venv
           "/opt/python/${PYTHON_TAG}-${PYTHON_TAG}/bin/python" -m venv "${VENV}"
+          "${VENV}/bin/python" -m pip install -U pip
           wheel="$(echo /work/dist/*.whl)"
           "${VENV}/bin/python" -m pip install "${wheel}[test]"