python2.7 pip requires updated get-pip.py URL

[Grady Phillips]

The Python website has updated the get-pip.py links for use for versions Python3+. Using the old URL makes builds on certain containers fail (my first being beckley).

Old: https://bootstrap.pypa.io/pip/get-pip.py
New (for Python2.7): https://bootstrap.pypa.io/pip/2.7/get-pip.py

Error log for reference (irrelevant output not included)

#5 134.6   % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
#5 134.6                                  Dload  Upload   Total   Spent    Left  Speed
  0     0    0     0    0     0      0      0 -100 1892k  100 1892k    0     0  5565k      0 --:--:-- --:--:-- --:--:-- 5565k
#5 134.9 ERROR: This script does not work on Python 2.7 The minimum supported Python version is 3.6. Please use https://bootstrap.pypa.io/pip/2.7/get-pip.py instead.
#5 134.9 /bin/sh: 1: pip: not found
#5 134.9 /bin/sh: 1: pip: not found
------
executor failed running [/bin/sh -c apt-get update -y;     apt-get install -y python curl python-dev git libssl-dev libffi-dev build-essential;     curl https://bootstrap.pypa.io/get-pip.py -o get-pip.py;     python get-pip.py;     pip install skyfield;    pip install --upgrade pwntools]: exit code: 127
make: *** [Makefile:14: solver] Error 1