Create new variable DRUPAL_PROJECTS_PATH and use it to remove hardcoded "/custom" paths

Migrated issue

Reported by: fjgarlin

Related to !334 (merged)

Problem/Motivation

After #3505585: Create environment variable for the project's own folder, we could see how "/custom" still appears in some places in the code. It'd be great if we can convert those instances to calculated paths from DRUPAL_PROJECT_FOLDER.

Proposed resolution

Calculate some of the values needed as parameters from the DRUPAL_PROJECT_FOLDER variable.
Some suggestions can be found here.

We could make the destination sub-path a variable, so for D10+ would be modules/custom and for D7 sites/all/modules/custom. This way we'd have all the elements of the DRUPAL_PROJECT_FOLDER variable broken down into reusable variables:

variables:
  DRUPAL_PROJECTS_PATH: "modules/custom"

export DRUPAL_PROJECT_FOLDER=$CI_PROJECT_DIR/$_WEB_ROOT/$DRUPAL_PROJECTS_PATH/$CI_PROJECT_NAME

D10 changes check-list

• new variable written to build.env (default and modified)
• environment vars (default and modified)
• composer symlink parameter (default and modified)
• eslint links to config files (default and modified)
• phpunit with _PHPUNIT_CONCURRENT:1 and _PHPUNIT_TESTGROUPS: --all (default, modified, module-nnnnn)
• upgrade status sed (default and modified)

D7 changes check-list

• new variable written to build.env (default and modified)
• environment vars (default and modified)
• composer symlink parameter (default and modified)
• ls directories at end of composer job (default and modified)
• phpunit adding /$CI_PROJECT_NAME (default, modified, module-nnnnn)

Related issue: Issue #3460115

Related issue: Issue #3505585