Just: read python_version from env for nested just propagation

Use env("python_version", "3") so that when the parent just process
exports the variable, nested just calls (via language_tests.py) pick
it up.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: redsun82

python/ql/justfile

@@ -5,7 +5,7 @@ format *ARGS=".": (_format_ql ARGS)
 
 consistency_queries := source_dir() / "consistency-queries"
 
-python_version := "3"
+python_version := env("python_version", "3")
 
 _v2_env := "CODEQL_EXTRACTOR_PYTHON_ANALYSIS_VERSION=2 CODEQL_PYTHON_LEGACY_TEST_EXTRACTION_VERSION=2"
 _v3_env := "CODEQL_PYTHON_LEGACY_TEST_EXTRACTION_VERSION=3"