Ignore platform requirements via environment variable

[maikschneider]

To make sure the renovate bot does not update to a dependency version that does not apply with the platform environment (e.g. php version), the --ignore-platform-reqs option should be removed from the composer install command.
However, there are cases where special requirements are not fulfilled (e.g. ext-ldap). To ignore these requirements, a new environment variable $COMPOSER_INSTALL_OPTIONS is introduced.

Usage:

variables:
  COMPOSER_INSTALL_OPTIONS: "--ignore-platform-req=ext-ldap"

include:
  - 'https://raw.githubusercontent.com/xima-media/gitlab-templates/main/build-php.yml'