Pin Ubuntu version to 20.04 in Tests and Tests (Storage with server)

[HideakiImamura]

Motivation

Fix the CI error in the master. According to actions/setup-python#401 (comment), we should add an environment variable AGENT_TOOLSDIRECTORY. The error seems to be fixed in my forked repo. This is a fix for the self-hosted runners. Sorry for the confusion.

The current CI of the master branch fails due to the installation fail of Python 3.6. This is because that the ubuntu-latest has been changed from 20.04 to 22.04. So, I will pin the version to 20.04 in the failing workflow: Tests and Tests (Storage with server).

Description of the changes

• Pin ubuntu version to 20.04 in the CI workflows Tests and Tests (Storage with server).

[HideakiImamura]

Sorry, the above issue and solution is for the self-hosted runners.

[codecov-commenter]

Codecov Report

Merging #4118 (6ee95a3) into master (a879936) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #4118   +/-   ##
=======================================
  Coverage   90.09%   90.09%           
=======================================
  Files         161      161           
  Lines       12655    12655           
=======================================
  Hits        11401    11401           
  Misses       1254     1254           

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more

[HideakiImamura]

I have decided to remove the Python 3.6 from the Tests and Tests (Storage with server) to pass the CI, since I have no idea why Python 3.6 cannot be installed. This will be investigated afterward. I will create an issue.

[HideakiImamura]

Let me change the PR title and description to match the current status of this PR.

[c-bata]

FYI: I've just opened #4119 to fix https://github.com/optuna/optuna/actions/runs/3392185067/jobs/5638086168

[not522]

LGTM. ubuntu-latest dropped Python 3.6 support. actions/runner-images#6399

[c-bata]

Thanks for the update!