Use RbConfig::CONFIG["rubylibdir"] to check for stdlib files

Ref: #431

CHANGELOG.md

@@ -1,5 +1,7 @@
 # Unreleased
 
+* Use `RbConfig::CONFIG["rubylibdir"]` instead of `RbConfig::CONFIG["libdir"]` to check for stdlib files. See #431.
+
 # 1.15.0
 
 * Add a readonly mode, for environments in which the updated cache wouldn't be persisted. See #428 and #423.

lib/bootsnap/load_path_cache/path.rb

@@ -116,8 +116,8 @@ def latest_mtime(path, dirs)
       VOLATILE = :volatile
 
       # Built-in ruby lib stuff doesn't change, but things can occasionally be
-      # installed into sitedir, which generally lives under libdir.
-      RUBY_LIBDIR  = RbConfig::CONFIG["libdir"]
+      # installed into sitedir, which generally lives under rubylibdir.
+      RUBY_LIBDIR  = RbConfig::CONFIG["rubylibdir"]
       RUBY_SITEDIR = RbConfig::CONFIG["sitedir"]
 
       def stability

test/load_path_cache/path_test.rb

@@ -16,7 +16,7 @@ def test_stability
         volatile        = Path.new(__FILE__)
         stable          = Path.new(time_file)
         unknown         = Path.new("/who/knows")
-        lib             = Path.new("#{RbConfig::CONFIG['libdir']}/a")
+        lib             = Path.new("#{RbConfig::CONFIG['rubylibdir']}/a")
         site            = Path.new("#{RbConfig::CONFIG['sitedir']}/b")
         absolute_prefix = RbConfig::CONFIG["host_os"] =~ /mswin|mingw|cygwin/ ? ENV["SystemDrive"] : ""
         bundler         = Path.new("#{absolute_prefix}/bp/3")