Custom runner for global task

[max-heller]

Currently, the global task is always run on ubuntu-20.04, but a global artifact job that I run needs a newer version (e.g. ubuntu-latest). Would it be possible to add an override for the global task runner similar to how custom runners are supported for other jobs (#614)?

cargo-dist/cargo-dist/src/backend/ci/github.rs

Lines 100 to 113 in 72d2bd5

	// The global task is responsible for:
	//
	// 1. building "global artifacts" like platform-agnostic installers
	// 2. stitching together dist-manifests from all builds to produce a final one
	//
	// If we've done a Good Job, then these artifacts should be possible to build on *any*
	// platform. Linux is usually fast/cheap, so that's a reasonable choice.
	let global_task = GithubMatrixEntry {
	targets: None,
	runner: Some(GITHUB_LINUX_RUNNER.into()),
	dist_args: Some("--artifacts=global".into()),
	install_dist: Some(install_dist_sh.clone()),
	packages_install: None,
	};

[max-heller]

Addressed by #1055