tf-m: hafnium: set an explicit pip version

fixes the following error:

  File "/tmp/get-pip.py", line 199, in main
    bootstrap(tmpdir=tmpdir)
  File "/tmp/get-pip.py", line 82, in bootstrap
    from pip._internal.cli.main import main as pip_entry_point
  File "/tmp/tmpcP1hEo/pip.zip/pip/_internal/cli/main.py", line 60
    sys.stderr.write(f"ERROR: {exc}")
                                   ^
SyntaxError: invalid syntax

Signed-off-by: Fathi Boudra <fathi.boudra@linaro.org>
Change-Id: I4895c8db072a8878325b022d8f9cb5e4d50d00ee
2 files changed