fix: don't set sibling directory for venvs output directory

[mattem]

Don't set the sibling attribute on the venv directory when creating it in the action. This fixes the error

Error in declare_directory: the output directory 'py/tests/external-deps/custom-macro/click_sibling_binary.venv.source' is not under package directory 'py/tests/external-deps/sibling-package' for target '//py/tests/external-deps/sibling-package:click_sibling_binary'

When referencing a source file that is referenced in a rule is referenced outside of that package or its direct ancestors. A way to repro and test this is create a simple macro that references the __main__.py file from where the macro is, rather than where it's called.