ci: cache dependencies in worker pipelines

Description

Cache pip installed packages in major pipelines which get invalidated if setup.py changes or date is different.
Test run pipelines - on average saved over 1min pipeline runtime per run

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.

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 84.89%. Comparing base (0913d9c) to head (8c73d1a).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## dev #1467 +/- ## ========================================== - Coverage 85.23% 84.89% -0.35% ========================================== Files 35 35 Lines 1998 1986 -12 Branches 378 293 -85 ========================================== - Hits 1703 1686 -17 - Misses 219 224 +5 Partials 76 76 ``` | [Flag](https://app.codecov.io/gh/Azure/azure-functions-python-worker/pull/1467/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Azure) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/Azure/azure-functions-python-worker/pull/1467/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Azure) | `84.89% <ø> (-0.30%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Azure#carryforward-flags-in-the-pull-request-comment) to find out more.

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Azure / azure-functions-python-worker