fix: Install zip from apt on Debian dev container bases

[leocavalcante]

Summary

• The feature always ran mlocati/docker-php-extension-installer 2.2.5. That script exits unless docker-php-ext-install and the other official PHP image helpers are on PATH. mcr.microsoft.com/devcontainers/base:bullseye has none of them, so zip never started installing.
• Official PHP images still use that installer. Debian and Ubuntu images install php-cli and the distro package with apt. zip installs php-zip.
• Other images fail before the upstream script is downloaded. The error names both supported bases.

Test plan

• bash tests/install-php-extensions.sh
• shellcheck on features/install-php-extensions/install.sh and tests/install-php-extensions.sh
• Build mcr.microsoft.com/devcontainers/base:bullseye with extensions set to zip and confirm php -m lists zip
• Build an official php image with the same option and confirm the mlocati installer still runs

Fixes #2