Update to RunnerSet, use concurrency control

[Nuru]

what

• Migrate arm64 workflows to self-hosted Runner Set (from older-style Runners)
• Add concurrency control to kill builds that are superseded
• When overriding package version for build, set package "release" to 0 (zero) if not also overridden

why

• Better autoscaling
• With PRs like this, all packages rebuild, and if there is a problem, they all get rebuilt, and the earlier builds get thrown away. Using concurrency control with preemption, we don't wast resources on finishing builds that will not be used.
• Previously, "release" was taken from RELEASE if not overridden. When overriding VERSION, presumably to fill in a missing older version, it does not make sense to use the package release number for the current version.