ci: run pip as module for all os

[ydcjeff]

Description:
Fixes fail to get python wheel cache in ubuntu CI.
ubuntu has been updated to 20.04, we can just use pip
For to work with cross platform, we need to with python3 -m as macos still has python2.7

Check list:

• New tests are added (if a new feature is added)
• New doc strings: description and/or example code are in RST format
• Documentation is updated (if required)

[trsvchn]

@ydcjeff Thank you for your PR. I am just wondering about windows, since I was facing this kind of issue when running python3 -m venv venv on windows-latest:

/c/hostedtoolcache/windows/Python/3.7.9/x64/python3
Error: [WinError 2] The system cannot find the file specified

See: msys2/MINGW-packages#5001 (comment)

So, I am not sure about pip, maybe it was venv issue or smth, did you try it on windows-latest?

[ydcjeff]

If use with setup-python, python alone is enough and it works

[vfdev-5]

Thanks @ydcjeff