Fix warning about CodeQL CODEQL_PYTHON variable

[danyeaw]

CodeQL runs are giving us the following warning:

We recommend that you remove the CODEQL_PYTHON environment variable from your workflow. This environment variable was originally used to specify a Python executable that included the dependencies of your Python code, however Python analysis no longer uses these dependencies.
If you used CODEQL_PYTHON to force the version of Python to analyze as, please use CODEQL_EXTRACTOR_PYTHON_ANALYSIS_VERSION instead, such as 'CODEQL_EXTRACTOR_PYTHON_ANALYSIS_VERSION=2.7' or 'CODEQL_EXTRACTOR_PYTHON_ANALYSIS_VERSION=3.11'.

Since we were using this variable to force the use of the poetry Python, this PR removes use of it.

PR Checklist

Please check if your PR fulfills the following requirements:

• I have read, and I am following the Contributor guide
• I have read, and I understand the GNOME Code of Conduct

PR Type

What kind of change does this PR introduce?

• Bug fix
• Feature
• Chore (refactoring, formatting, local variables, other cleanup)
• Documentation content changes

What is the current behavior?

Warnings during CodeQL builds

Issue Number: N/A

What is the new behavior?

No CodeQL warnings during builds

Does this PR introduce a breaking change?

• Yes
• No

Other information