Provide enough information to the script trough the use of a context

[ereOn]

When doing the template rendering, we must use a context that contains:

• The current matrix variant
• Global variables
• Plix's specific execution variables (.plix.yml file's location, current build path (see Build directory creation #1))

Note that to prevent name-clashing, plix's specific execution variables should belong to a named variable (likely plix) such that one can access it that way:

script:
  - echo {{plix.plix_source_path}}
  - echo {{plix.plix_build_path}}