fix: bootstrap pip in venv (ensurepip not available on RISE runners)

Author: gounthar

.github/workflows/build-riscv64.yml

@@ -29,8 +29,9 @@ jobs:
       - name: Set up Python
         run: |
           export PATH="$(ls -d /opt/python-3.1*/bin 2>/dev/null | sort -V | tail -1):$PATH"
-          python3 -m venv /tmp/build-venv
+          python3 -m venv --without-pip /tmp/build-venv
           . /tmp/build-venv/bin/activate
+          curl -sS https://bootstrap.pypa.io/get-pip.py | python3
           pip install --upgrade pip setuptools wheel
 
       - name: Build wheel