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 @@
 const fs = require('fs')
+const os = require('os')
 const path = require('path')
 const core = require('@actions/core')
 const exec = 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
+  const jobs = Math.min(os.cpus().length, 8)
   // Always run 'bundle install' to list the gems
-  await exec.exec('bundle', ['install', '--jobs', '4'])
+  await exec.exec('bundle', ['install', '--jobs', jobs])
 
   // @actions/cache only allows to save for non-existing keys
   if (!common.isExactCacheKeyMatch(key, cachedKey)) {