fix: set permissions for /usr/local/bin

[stasadev]

The Issue

Cannot change Node.js version:

$ ddev config --nodejs-version=v18
$ ddev start

$ ddev exec node -v
v24.12.0

$ ddev logs
...
+ n-install.sh
This script cannot write to the directory /usr/local/bin
...

How This PR Solves The Issue

COPY --from=python:2.7-slim /usr/local /usr/local resets directory permissions, this PR adds it back, along with test coverage.

Manual Testing Instructions

ddev add-on get https://github.com/ddev/ddev-python2/tarball/refs/pull/16/head
ddev config --nodejs-version=v18
ddev restart
ddev exec python --version
ddev exec pip --version
ddev exec node -v

Automated Testing Overview

Release/Deployment Notes