Document the logic for whether to install Bundler

https://github.com/actions/virtual-environments/blob/dee50eebffa60a7dd9149033675d44a027dd6b33/README.md#available-environments

README.md

@@ -38,7 +38,7 @@ The action works for all [GitHub-hosted runners](https://help.github.com/en/acti
 
 | Operating System | Recommended | Other Supported Versions |
 | ----------- | -------- | -------- |
-| Ubuntu  | `ubuntu-latest`  (= `ubuntu-18.04`) | `ubuntu-20.04`, `ubuntu-16.04` |
+| Ubuntu  | `ubuntu-latest`  (= `ubuntu-20.04`) | `ubuntu-18.04`, `ubuntu-16.04` |
 | macOS   | `macos-latest`   (= `macos-10.15`)  | `macos-11.0` |
 | Windows | `windows-latest` (= `windows-2019`) | `windows-2016` |
 
@@ -83,6 +83,7 @@ jobs:
       fail-fast: false
       matrix:
         os: [ubuntu-latest, macos-latest]
+        # Due to https://github.com/actions/runner/issues/849, we have to use quotes for '3.0'
         ruby: [2.5, 2.6, 2.7, '3.0', head, jruby, jruby-head, truffleruby, truffleruby-head]
     runs-on: ${{ matrix.os }}
     steps:
@@ -110,7 +111,7 @@ jobs:
       BUNDLE_GEMFILE: gemfiles/${{ matrix.gemfile }}.gemfile
     steps:
       - uses: actions/checkout@v2
-      - uses: ./
+      - uses: ruby/setup-ruby@v1
         with:
           ruby-version: 2.6
           bundler-cache: true # runs 'bundle install' and caches installed gems automatically

bundler.js

@@ -81,6 +81,8 @@ export async function installBundler(bundlerVersionInput, lockFile, platform, ru
   }
 
   if (common.isHeadVersion(rubyVersion) && common.isBundler2Default(engine, rubyVersion) && bundlerVersion.startsWith('2')) {
+    // Avoid installing a newer Bundler version for head versions as it might not work.
+    // For releases, even if they ship with Bundler 2 we install the latest Bundler.
     console.log(`Using Bundler 2 shipped with ${engine}-${rubyVersion}`)
   } else if (engine === 'truffleruby' && !common.isHeadVersion(rubyVersion) && bundlerVersion.startsWith('1')) {
     console.log(`Using Bundler 1 shipped with ${engine}`)

common.js

@@ -59,7 +59,9 @@ export function isBundler2Default(engine, rubyVersion) {
   if (engine === 'ruby') {
     return isHeadVersion(rubyVersion) || floatVersion(rubyVersion) >= 2.7
   } else if (engine === 'truffleruby') {
-    return isHeadVersion(rubyVersion)
+    return isHeadVersion(rubyVersion) || floatVersion(rubyVersion) >= 21.0
+  } else if (engine === 'jruby') {
+    return isHeadVersion(rubyVersion) || floatVersion(rubyVersion) >= 9.3
   } else {
     return false
   }

dist/index.js

@@ -32851,7 +32851,9 @@ function isBundler2Default(engine, rubyVersion) {
   if (engine === 'ruby') {
     return isHeadVersion(rubyVersion) || floatVersion(rubyVersion) >= 2.7
   } else if (engine === 'truffleruby') {
-    return isHeadVersion(rubyVersion)
+    return isHeadVersion(rubyVersion) || floatVersion(rubyVersion) >= 21.0
+  } else if (engine === 'jruby') {
+    return isHeadVersion(rubyVersion) || floatVersion(rubyVersion) >= 9.3
   } else {
     return false
   }
@@ -44617,6 +44619,8 @@ async function installBundler(bundlerVersionInput, lockFile, platform, rubyPrefi
   }
 
   if (common.isHeadVersion(rubyVersion) && common.isBundler2Default(engine, rubyVersion) && bundlerVersion.startsWith('2')) {
+    // Avoid installing a newer Bundler version for head versions as it might not work.
+    // For releases, even if they ship with Bundler 2 we install the latest Bundler.
     console.log(`Using Bundler 2 shipped with ${engine}-${rubyVersion}`)
   } else if (engine === 'truffleruby' && !common.isHeadVersion(rubyVersion) && bundlerVersion.startsWith('1')) {
     console.log(`Using Bundler 1 shipped with ${engine}`)