Install Bundler at runtime if not already part of the stdlib

* This gives more flexibility to choose the Bundler version.

index.js

@@ -2,6 +2,7 @@ const os = require('os')
 const fs = require('fs')
 const path = require('path')
 const core = require('@actions/core')
+const exec = require('@actions/exec')
 
 export async function run() {
   try {
@@ -23,6 +24,9 @@ export async function run() {
     const [rubyPrefix, newPathEntries] = await installer.install(platform, ruby)
 
     setupPath(ruby, newPathEntries)
+
+    await installBundler(platform, rubyPrefix)
+
     core.setOutput('ruby-prefix', rubyPrefix)
   } catch (error) {
     core.setFailed(error.message)
@@ -107,6 +111,14 @@ function setupPath(ruby, newPathEntries) {
     core.exportVariable('PATH', [...newPathEntries, ...cleanPath].join(path.delimiter))
 }
 
+async function installBundler(platform, rubyPrefix) {
+  const bundle_exe = platform === 'windows-latest' ? 'bundle.cmd' : 'bundle'
+  // Install Bundler if not already part of the stdlib
+  if (!fs.existsSync(path.join(rubyPrefix, 'bin', bundle_exe))) {
+    await exec.exec(path.join(rubyPrefix, 'bin', 'gem'), ['install', 'bundler', '-v', '~> 1', '--no-document'])
+  }
+}
+
 function getVirtualEnvironmentName() {
   const platform = os.platform()
   if (platform === 'linux') {

windows.js

@@ -50,11 +50,6 @@ export async function install(platform, ruby) {
     await setupMSWin() : await setupMingw(version)
   const newPathEntries = [`${rubyPrefix}\\bin`, ...toolchainPaths]
 
-  // Install Bundler if needed
-  if (!fs.existsSync(`${rubyPrefix}\\bin\\bundle.cmd`)) {
-    await exec.exec(`${rubyPrefix}\\bin\\gem install bundler -v "~> 1" --no-document`)
-  }
-
   return [rubyPrefix, newPathEntries]
 }