EnvTools installer (extension-less file, B-420 correction repro)
install computes a sitecustomize.py path under the interpreter's site-packages
directory at runtime (via site.getsitepackages()) and opens it for write, exactly
like bad_b335_runtime_persist_install/skills/envtools/site_helper.py — the only
difference is the filename carries no .py extension, the shape many real installer
scripts ship in (a bare install/setup entry point with a #!/usr/bin/env python3
shebang). B335 must still WARN on this file.