Export downstream test group input

[ChrisRackauckas-Claude]

This PR should be ignored until reviewed by @ChrisRackauckas.

Summary

• Exports the reusable downstream workflow group input as GROUP for the Julia test step.
• This lets downstream packages that dispatch on ENV["GROUP"] actually run the requested group instead of their default/full suite.

Context

SciML/.github/.github/workflows/downstream.yml@v1 accepts a group input and uses it in the job name, but the Julia test step does not currently set GROUP. In NonlinearSolve downstream CI this made the ModelingToolkit jobs labelled InterfaceI, InterfaceII, and Initialization continue beyond the requested group and run additional groups in the same process.

Local verification

• git diff --check
  • Result: passed
• ruby -e 'require "yaml"; YAML.load_file(".github/workflows/downstream.yml"); puts "downstream.yml parsed"'
  • Result: downstream.yml parsed

Note

Callers pinned to SciML/.github/.github/workflows/downstream.yml@v1 will need the v1 tag/reference updated after this lands to consume the fix.

[ChrisRackauckas-Claude]

Scratchpad update: local YAML parsing and git diff --check passed, and the PR CI is green (actionlint + shellcheck, compute_affected_sublibraries.jl tests). The remaining deployment dependency is advancing the floating v1 reference after this lands so callers using downstream.yml@v1 consume the exported GROUP fix.