Mirrors Nixpacks behavior: inject COOLIFY_* and SOURCE_COMMIT into
railpack build variables so apps (e.g. SPAs baking public URLs) can
read them via /run/secrets/<KEY>.
Rename is_nixpacks → is_buildpack_control to cover both NIXPACKS_ and
RAILPACK_ prefixed keys. Update the env variable view and appends list
accordingly.
Promote generate_coolify_env_variables to protected for testability.
b6ca6b1b20