Scale bundler install jobs with CPU count up to 8

The number of jobs should scale with the runner CPU count, up to a point. The bottleneck becomes network / disk I/O at very large numbers, and 8 seems like a reasonable maximum based on my experience at this time.

Author: thomasbreland

bundler.js

@@ -1,4 +1,5 @@
 constfs=require('fs')
+constos=require('os')
 constpath=require('path')
 constcore=require('@actions/core')
 constexec=require('@actions/exec')
@@ -193,8 +194,10 @@ export async function bundleInstall(gemfile, lockFile, platform, engine, rubyVer
 console.log(`Found cache for key: ${cachedKey}`)
 }
 
+// Number of jobs should scale with runner, up to a point
+constjobs=Math.min(os.cpus().length,8)
 // Always run 'bundle install' to list the gems
-awaitexec.exec('bundle',['install','--jobs','4'])
+awaitexec.exec('bundle',['install','--jobs',jobs])
 
 // @actions/cache only allows to save for non-existing keys
 if(!common.isExactCacheKeyMatch(key,cachedKey)){