Use older pip version to remain compatible with Python 3.7

Fixes BuildBundle-Linux as upstream change dropped support for Python
3.7.

.github/workflows/release.yml

@@ -108,7 +108,7 @@ jobs:
             build-essential bash-completion curl lsb-release sudo g++ gcc flex \
             bison make patch git python3.7 python3.7-dev python3.7-distutils
           update-alternatives --install /usr/bin/python3 python3 /usr/bin/python3.7 1
-          curl -s https://bootstrap.pypa.io/get-pip.py -o get-pip.py
+          curl -s https://bootstrap.pypa.io/pip/3.7/get-pip.py -o get-pip.py
           python3 get-pip.py --force-reinstall
           rm get-pip.py