Upgrade Linux pip before packaging worker

[Hazhzeng]

Description

There's a size increase (~90MB) in grpcio library for Linux Python 3.6 worker which is caused by the manylinux2010 wheel is only recognized with pip >= 19.0.0.

I add the linux pip upgrade command in our linux package script.
Windows package script already has it.

---

PR information

• The title of the PR is clear and informative.
• There are a small number of commits, each of which has an informative message. This means that previously merged commits do not appear in the history of the PR. For information on cleaning up the commits in your pull request, see this page.
• If applicable, the PR references the bug/issue that it fixes in the description.
• New Unit tests were added for the changes made and CI is passing.

Quality of Code and Contribution Guidelines

• I have read the contribution guidelines.