Use os.availableParallelism() instead of os.cpus().length

* As https://nodejs.org/api/os.html#oscpus says:
  os.cpus().length should not be used to calculate the amount of parallelism available to an application. Use os.availableParallelism() for this purpose.

Author: eregon

bundler.js

@@ -195,7 +195,7 @@ export async function bundleInstall(gemfile, lockFile, platform, engine, rubyVer
   }
 
   // Number of jobs should scale with runner, up to a point
-  const jobs = Math.min(os.cpus().length, 8)
+  const jobs = Math.min(os.availableParallelism(), 8)
   // Always run 'bundle install' to list the gems
   await exec.exec('bundle', ['install', '--jobs', jobs])