Fork pull requests, the nightly run, Run workflow, the test archive and
test shards 3-4 move from ubuntu-24.04 to blacksmith-<N>vcpu-ubuntu-2404
(16 vCPU for Clippy and the test archive, 8 for test shards, doc tests,
reference tests and cross-compiles). Trusted runs keep the self-hosted
routing. Blacksmith runners count as self-hosted to GitHub, so steps now
tell our own machines apart by runner name.
TEMPORARY: this branch routes its own run like a fork pull request.
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>
Co-authored-by: jyviko <2501648+jyviko@users.noreply.github.com>
