Create change_dir before running the external package build command - #4095

The external package builder is the only one of the four change_dir call sites that does not create the directory before running commands in it.

The documentation for the external builder's change_dir states the opposite:

Change to this working directory when executing the package build command. If the directory does not exist yet, it will be created (required for Windows to be able to execute any command).

The three siblings all do it, one line above their run_command_set call:

Without it, pointing change_dir at a directory that does not exist yet fails the whole run from the subprocess layer:

.pkg_external: Exception running subprocess [Errno 2] No such file or directory: '.../build-here'
py: FAIL code 1

while the same change_dir on a run env creates the directory and works.

The fix is the same line the siblings use. The added test asserts the directory exists after the run, and it fails on main with assert PosixPath('.../build-here').is_dir().

tests/tox_env/python/virtual_env/package/test_package_cmd_builder.py is 11 passed, 2 skipped (the two integration cases). ruff check and ruff format --check are clean with the 0.16.8 pinned in .pre-commit-config.yaml.

I will add the docs/changelog/ entry as soon as this PR has a number.