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.

dist/index.js

@@ -13,6 +13,7 @@ __nccwpck_require__.r(__webpack_exports__);
 /* harmony export */   installBundler: () => (/* binding */ installBundler)
 /* harmony export */ });
 const fs = __nccwpck_require__(9896)
+const os = __nccwpck_require__(857)
 const path = __nccwpck_require__(6928)
 const core = __nccwpck_require__(7484)
 const exec = __nccwpck_require__(5236)
@@ -207,8 +208,10 @@ async function bundleInstall(gemfile, lockFile, platform, engine, rubyVersion, b
     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)) {