ci: Change install nox command to use the default python.

[SinaChavoshi]

This PR is to address a test failure issue with Kokoro test infra.

Changing the python version used to python3 instead of python3.6

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Fixes #<issue_number_goes_here> 🦕

[conventional-commit-lint-gcf]

🤖 I detect that the PR title and the commit message differ and there's only one commit. To use the PR title for the commit history, you can use Github's automerge feature with squashing, or use automerge label. Good luck human!

-- conventional-commit-lint bot
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