Add pip cache for GitHub Actions

[drew2a]

This PR adds cache for pip python dependencies. It should decrease the time of job runs.

(this improvement saves ~1m for each run)

Ref:

• PIP cache should cache the installed packages as well actions/setup-python#330
• https://docs.github.com/en/actions/creating-actions/about-custom-actions#composite-actions
• https://github.com/syphar/restore-virtualenv

[xoriole]

Nice! How can the cache be cleared if we need to?

[drew2a]

@xoriole in the current implementation it is not possible.

But for cache renewal like this:

python -m pip install --upgrade pip
pip install -r requirements.txt

You can just comment this line:
https://github.com/drew2a/tribler/blob/a48031e768de099ec2ad955051bd3adc859e36b1/.github/actions/pyenv/action.yml#L33

[drew2a]

Also please take a look at this link: https://docs.github.com/en/actions/using-workflows/caching-dependencies-to-speed-up-workflows#usage-limits-and-eviction-policy