ci: distribute compatibility release tests

[crazy-max]

relates to:

System.IO.IOException: No space left on device : '/home/runner/actions-runner/cached/2.337.0/_diag/Worker_20260908-072835-utc.log'
   at System.IO.RandomAccess.WriteAtOffset(SafeFileHandle handle, ReadOnlySpan`1 buffer, Int64 fileOffset)
   at System.IO.StreamWriter.Flush(Boolean flushStream, Boolean flushEncoder)
   at System.Diagnostics.TextWriterTraceListener.Flush()
   at GitHub.Runner.Common.HostTraceListener.WriteHeader(String source, TraceEventType eventType, Int32 id)
   at System.Diagnostics.TraceSource.TraceEvent(TraceEventType eventType, Int32 id, String message)
   at GitHub.Runner.Worker.Worker.RunAsync(String pipeIn, String pipeOut)
   at GitHub.Runner.Worker.Worker.RunAsync(String pipeIn, String pipeOut)
   at GitHub.Runner.Worker.Program.MainAsync(IHostContext context, String[] args)

The compatibility releases workflow currently runs every historical BuildKit release test on a single GitHub-hosted runner. This can exhaust the runner disk while the job is still writing runner diagnostics, which showed up as System.IO.IOException: No space left on device.

This change distributes the release list across a workflow matrix and passes one release:compatibility-version entry to each job through the existing COMPATIBILITY_RELEASES input. Each compatibility release now runs on its own runner, reducing per-runner disk pressure while preserving the same test coverage.