Change get-pip download URL to pin version

When I tried to run the install_bb_python.sh script it failed with an error

+ ./python3 get-pip/get-pip.py
Traceback (most recent call last):
  File "get-pip/get-pip.py", line 24244, in <module>
    main()
  File "get-pip/get-pip.py", line 199, in main
    bootstrap(tmpdir=tmpdir)
  File "get-pip/get-pip.py", line 82, in bootstrap
    from pip._internal.cli.main import main as pip_entry_point
  File "<frozen importlib._bootstrap>", line 969, in _find_and_load
  File "<frozen importlib._bootstrap>", line 954, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 896, in _find_spec
  File "<frozen importlib._bootstrap_external>", line 1136, in find_spec
  File "<frozen importlib._bootstrap_external>", line 1112, in _get_spec
  File "<frozen importlib._bootstrap_external>", line 1093, in _legacy_get_spec
  File "<frozen importlib._bootstrap>", line 444, in spec_from_loader
  File "<frozen importlib._bootstrap_external>", line 530, in spec_from_file_location
  File "/tmp/tmpiypbez55/pip.zip/pip/_internal/cli/main.py", line 60
    sys.stderr.write(f"ERROR: {exc}")
                                   ^
SyntaxError: invalid syntax

With this help this ref and pip dox I figured out that you need to pin the version, and, of course, you don't need to clone but can just download that one file.