Scoped: route only apt-packages/container legs to GitHub-hosted

Note: please ignore until reviewed by @ChrisRackauckas.

Re-scope of the now-closed #96 (which proposed pinning the default runner to ubuntu-24.04). That was wrong — it would have moved the whole SciML fleet's CI onto GitHub-hosted runners, which lack the capacity for SciML's throughput. #96 could not be reopened (its branch was force-pushed after it was closed), so this PR — on the same branch default-runner-github-hosted — is its live successor.

Runner facts (unchanged)

The SciML self-hosted pool (demeter*/arctic*, ephemeral *-cxnps-*) is registered with the custom label ubuntu-latest — the same label GitHub-hosted runners answer. Its full label set is {self-hosted, Linux, X64, gpu, high-memory, ubuntu-latest}; it does not carry the pinned ubuntu-24.04 label. So:

• ubuntu-latest → self-hosted-capable (kept as the default for throughput).
• ubuntu-24.04 → GitHub-hosted only (has passwordless sudo + docker).

What this changes

Keep ubuntu-latest as the default for normal test/downgrade legs, and force GitHub-hosted only for the legs that genuinely need passwordless sudo / docker — i.e. exactly the legs where the caller passes apt-packages or a container. The persistent demeter*/arctic* runners lack passwordless sudo, so sudo apt-get (apt provisioning) intermittently fails with sudo: a terminal is required to read the password whenever such a leg lands there (ChrisRackauckas/InternalJunk#52); containers likewise need a Docker host.

Conditional added at the reusable job's runs-on:

runs-on: ${{ (inputs.apt-packages != '' || inputs.container != '') && fromJSON('["ubuntu-24.04"]') || <existing default> }}

fromJSON('["ubuntu-24.04"]') is a non-empty (truthy) array, so the GitHub Actions &&/|| ternary does not fall through; the default branch returns the existing value (an array via fromJson(inputs.runner), or a string 'self-hosted' / inputs.os / 'ubuntu-latest'). Both branches are valid runs-on forms (array or string) — the same idiom tests.yml already used for the runner/os selection. actionlint validates it (exit 0).

Where it landed

The reusables that accept apt-packages/container and have a direct job-level runs-on:

grouped-tests.yml and sublibrary-project-tests.yml route their matrices through tests.yml (passing apt-packages/container through), so the single conditional in tests.yml covers them.

Reverted from #96 / left unchanged on purpose

• scripts/compute_affected_sublibraries.jl default runner and the os defaults in tests.yml/downgrade.yml are back to ubuntu-latest.
• detect/discover helper jobs (grouped-tests.yml, sublibrary-project-tests.yml, sublibrary-downgrade.yml discover) stay ubuntu-latest.
• sublibrary-project-tests.yml exposes no apt-packages/container input and passes none through, so its matrix legs never need the override.
• Groups with an explicit self-hosted runner (GPU) or an OS-axis override set no apt-packages/container, so they stay self-hosted / on their OS.

Affected repos

The only repos passing apt-packages/container today:

• SciPyDiffEq.jl — apt-packages: "python3-scipy"
• deSolveDiffEq.jl — apt-packages: "r-base-dev r-cran-desolve"
• FEniCS.jl — container: "cmhyett/julia-fenics:latest"

Tests

test/runtests.jl re-asserts the default matrix runner is ubuntu-latest, and a new runs-on conditional testset confirms the real expression is present in each reusable and (emulating GitHub Actions truthiness) that apt-packages/container resolve to ubuntu-24.04 while the default (incl. GPU self-hosted override) is preserved. Passing on Julia 1.10 and 1.12; actionlint clean. Live routing itself can only be proven by a retagged run.

Deploy

Needs a v1 retag to take effect fleet-wide.

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