CI: install Julia in FormatCheck workflow so runic-action can run

[ChrisRackauckas-Claude]

fredrikekre/runic-action@v1 (v1.4+) requires julia in PATH but no longer installs it itself. Add a julia-actions/setup-julia@v2 step before the action so the format check actually runs (it has been silently passing — i.e. silently failing — across many SciML repos for weeks).

Detected as part of the SciMLBase v3 / OrdinaryDiffEq v7 ecosystem rollout sweep.

Ignore until reviewed by @ChrisRackauckas.

[ChrisRackauckas-Claude]

Rebased fix-formatcheck-julia-setup onto current main to resolve the .github/workflows/FormatCheck.yml merge conflict.

Outcome: this PR is now obsolete and was auto-closed by GitHub (zero diff vs main).

Why: when this PR was opened, FormatCheck.yml had an inline runic job, and the fix was to add julia-actions/setup-julia@v2 before fredrikekre/runic-action@v1. Since then, main (via #57/#62 centralized-CI work) replaced that inline job with a thin caller:

jobs:
  runic:
    uses: "SciML/.github/.github/workflows/runic.yml@v1"
    secrets: "inherit"

The centralized reusable workflow SciML/.github/.github/workflows/runic.yml@v1 already installs Julia before running Runic (actions/checkout@v6 -> julia-actions/setup-julia@v3 -> fredrikekre/runic-action@v1). So the exact fix this PR intended — install Julia before the Runic step — is already in effect on main.

Resolving the conflict in favor of main's content yields a tree identical to main, leaving no commit for this PR to carry, so GitHub auto-closed it (closed, not merged; the change is already present on main). No action needed. Ignore unless re-review desired by @ChrisRackauckas.