Use BUNDLE_LOCKFILE when detecting the lockfile

Honor `BUNDLE_LOCKFILE` (added in Bundler 4) when selecting the lockfile used
for Bundler version detection, deployment mode, and bundler-cache keys. Without
this, workflows using an alternate lockfile can read the wrong `BUNDLED WITH`
version and generate cache keys from the wrong lockfile.

Author: Thomascountz

README.md

@@ -168,7 +168,7 @@ should be able to fix them by setting `rubygems: 3.0.0` or higher.
 
 By default, Bundler is installed as follows:
 
-* If there is a `Gemfile.lock` file (or `$BUNDLE_GEMFILE.lock` or `gems.locked`) with a `BUNDLED WITH` section,
+* If there is a `Gemfile.lock` file (or `$BUNDLE_LOCKFILE`, `$BUNDLE_GEMFILE.lock`, or `gems.locked`) with a `BUNDLED WITH` section,
   that version of Bundler will be installed and used.
 * If the Ruby ships with Bundler 2.2+ (as a default gem), that version is used.
 * Otherwise, the latest compatible Bundler version is installed (Bundler 2 on Ruby >= 2.3, Bundler 1 on Ruby < 2.3).
@@ -188,7 +188,7 @@ Note that any step doing `bundle install` (for the root `Gemfile`) or `gem insta
 
 This caching speeds up installing gems significantly and avoids too many requests to RubyGems.org.  
 It needs a `Gemfile` (or `$BUNDLE_GEMFILE` or `gems.rb`) under the [`working-directory`](#working-directory).  
-If there is a `Gemfile.lock` (or `$BUNDLE_GEMFILE.lock` or `gems.locked`), `bundle config --local deployment true` is used.
+If there is a `Gemfile.lock` (or `$BUNDLE_LOCKFILE`, `$BUNDLE_GEMFILE.lock`, or `gems.locked`), `bundle config --local deployment true` is used.
 
 To use a `Gemfile` which is not at the root or has a different name, set `BUNDLE_GEMFILE` in the `env` at the job level
 as shown in the [example](#matrix-of-gemfiles).

bundler.js

@@ -15,14 +15,16 @@ function isValidBundlerVersion(bundlerVersion) {
 // The returned gemfile is guaranteed to exist, the lockfile might not exist
 export function detectGemfiles() {
   const gemfilePath = process.env['BUNDLE_GEMFILE'] || 'Gemfile'
+  const lockfilePath = process.env['BUNDLE_LOCKFILE']
+
   if (fs.existsSync(gemfilePath)) {
-    return [gemfilePath, `${gemfilePath}.lock`]
+    return [gemfilePath, lockfilePath || `${gemfilePath}.lock`]
   } else if (process.env['BUNDLE_GEMFILE']) {
     throw new Error(`$BUNDLE_GEMFILE is set to ${gemfilePath} but does not exist`)
   }
 
   if (fs.existsSync("gems.rb")) {
-    return ["gems.rb", "gems.locked"]
+    return ["gems.rb", lockfilePath || "gems.locked"]
   }
 
   return [null, null]