[build] Fix the snmp docker build error. (sonic-net#7452)

Issue is get_pip.py is moved to pip 21.1 (https://github.com/pypa/get-pip/commits/main) which is not compatible with 3.6.
Issue of pip itself is fixed as part of 21.1.1 in pip community (pypa/pip#9835).
However get-pip.py is still not updated to latest pip. Also get.pip.py does not support python 3.6 version explicitly (pypa/get-pip#88)

Step 15/29 : RUN curl https://bootstrap.pypa.io/get-pip.py | python3.6
 ---> Running in bece31f49267
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed

  0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0
100 1891k  100 1891k    0     0  9564k      0 --:--:-- --:--:-- --:--:-- 9600k
Traceback (most recent call last):
  File "<stdin>", line 24298, in <module>
  File "<stdin>", line 139, in main
  File "<stdin>", line 115, in bootstrap
  File "<stdin>", line 96, in monkeypatch_for_cert
  File "/tmp/tmp5fnxrz0a/pip.zip/pip/_internal/commands/__init__.py", line 9, in <module>
  File "/tmp/tmp5fnxrz0a/pip.zip/pip/_internal/cli/base_command.py", line 12, in <module>
  File "/tmp/tmp5fnxrz0a/pip.zip/pip/_internal/cli/cmdoptions.py", line 30, in <module>
  File "/tmp/tmp5fnxrz0a/pip.zip/pip/_internal/utils/hashes.py", line 2, in <module>
ImportError: cannot import name 'NoReturn'
The command '/bin/sh -c curl https://bootstrap.pypa.io/get-pip.py | python3.6' returned a non-zero code: 1
How I did:

Got the file from https://github.com/pypa/get-pip/tree/21.0 and added to the buildimage
pin pip to the previous release 21.0.1. (Similar is done in other public repos eg: grpc/grpc-java#8115)

Signed-off-by: Abhishek Dosi <abdosi@microsoft.com>

dockers/docker-snmp-sv2/Dockerfile.j2

@@ -32,7 +32,8 @@ RUN dpkg_apt() { [ -f $1 ] && { dpkg -i $1 || apt-get -y install -f; } || return
 {%- endif %}
 
 # Install up-to-date version of pip
-RUN curl https://bootstrap.pypa.io/get-pip.py | python3.6
+COPY ["get-pip.py", "/usr/bin/"]
+RUN python3.6 /usr/bin/get-pip.py pip==21.0.1
 
 # pin down setuptools version for issues to install sonic_ax_impl
 # see https://github.com/Azure/sonic-buildimage/issues/5279