Pass require path to sub-process when running `bundle install` (#7618)

Merge pull request 7618

lib/jekyll/commands/new.rb

@@ -139,9 +139,11 @@ module Jekyll
         def after_install(path, options = {})
           unless options["blank"] || options["skip-bundle"]
             begin
-              require "bundler"
+              # Activate 'bundler' gem and puts it into the `loaded_specs`.
+              # For details: https://rubydocs.org/d/ruby-2-4-0/classes/Kernel.html#method-i-gem
+              gem "bundler"
               bundle_install path
-            rescue LoadError
+            rescue Gem::MissingSpecError
               Jekyll.logger.info "Could not load Bundler. Bundle install skipped."
             end
           end
@@ -153,8 +155,10 @@ module Jekyll
         def bundle_install(path)
           Jekyll.logger.info "Running bundle install in #{path.cyan}..."
           Dir.chdir(path) do
-            exe = Gem.bin_path("bundler", "bundle")
-            process, output = Jekyll::Utils::Exec.run("ruby", exe, "install")
+            bundler_gemspec = Gem.loaded_specs["bundler"]
+            exe = bundler_gemspec.bin_file "bundle"
+            require_paths = bundler_gemspec.full_require_paths
+            process, output = Jekyll::Utils::Exec.run("ruby", "-I", *require_paths, exe, "install")
 
             output.to_s.each_line do |line|
               Jekyll.logger.info("Bundler:".green, line.strip) unless line.to_s.empty?