Lint GitHub Actions workflows with actionlint

[mattgodbolt]

Part of the org-wide actionlint rollout (see compiler-explorer/compiler-workflows#68, compiler-explorer/infra#2226, compiler-explorer/compiler-explorer#8912). Checks workflow files for schema errors, expression-context mistakes, unknown runner labels, and shellcheck issues (errors+warnings; info/style gated in .github/actionlint.yaml) on any push/PR touching them.

Existing workflows already passed cleanly.

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Adds an actionlint workflow and repository configuration so GitHub Actions workflow files are linted (schema, expressions, runner labels, and embedded shell scripts via shellcheck) when workflow-related files change, aligning with the org-wide rollout referenced in the PR description.

Changes:

• Add a new workflow that runs actionlint on pushes/PRs that touch .github/workflows/** or .github/actionlint.yaml.
• Add .github/actionlint.yaml to define known self-hosted runner labels and to ignore shellcheck info/style findings in embedded scripts.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
.github/workflows/lint-actions.yml	New CI workflow to run actionlint when workflow/config files change.
.github/actionlint.yaml	actionlint configuration for runner labels and shellcheck info/style gating.

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[mattgodbolt]

Closing per review: we're keeping the actionlint rollout to the core repos (compiler-explorer, infra, compiler-workflows) rather than adding CI-only lint machinery to quiet single-workflow repos — all our other linters run as pre-commit/local hooks, and this repo's workflows were already clean anyway.