Removed deprecated echo set-env instruction for python code scanning excerpt

[AntoineRondelet]

Why:

While trying to set up code scanning on a python code base, I followed the workflow extract https://docs.github.com/en/free-pro-team@latest/github/finding-security-vulnerabilities-and-errors-in-your-code/configuring-code-scanning#analyzing-python-dependencies, however, I got the following error while trying to execute it:

Error: Unable to process command '::set-env name=CODEQL_PYTHON::my-path/python' successfully.
Error: The set-env command is disabled. Please upgrade to using Environment Files or opt into unsecure command execution by setting the ACTIONS_ALLOW_UNSECURE_COMMANDS environment variable to true. For more information see: https://github.blog/changelog/2020-10-01-github-actions-deprecating-set-env-and-add-path-commands/

After consulting the link provided, and after reading: https://docs.github.com/en/free-pro-team@latest/actions/reference/workflow-commands-for-github-actions#environment-files it seemed that the documentation was outdated (switching to echo "CODEQL_PYTHON=$(which python)" >> $GITHUB_ENV seems to resolve the issue on my end)

What's being changed:

As mentioned above, this PR changes:
echo "::set-env name=CODEQL_PYTHON::$(which python)" to echo "CODEQL_PYTHON=$(which python)" >> $GITHUB_ENV
Following https://github.blog/changelog/2020-10-01-github-actions-deprecating-set-env-and-add-path-commands/ and https://docs.github.com/en/free-pro-team@latest/actions/reference/workflow-commands-for-github-actions#environment-files

Check off the following:

• All of the tests are passing.
• I have reviewed my changes in staging. (look for the deploy-to-heroku link in your pull request, then click View deployment)
• For content changes, I have reviewed the localization checklist
• For content changes, I have reviewed the Content style guide for GitHub Docs.

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[janiceilene]

@AntoineRondelet Thanks so much for opening a PR! I'll get this triaged for review ⚡

[shati-patel]

Thank you so much for the fix @AntoineRondelet! I can confirm this is the correct syntax 🎉

I'll get your pull request merged shortly 😊

[github-actions]

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