Generate the bundle path before cleaning the env

When `Gem.bin_path("bundler", "bundle")` is used inside `with_unbundled_env`
it can return `nil`, thus breaking the command. By storing the path
outside of the block we ensure to grab a working path instead.

Also removed the unused `env` argument.

core/lib/generators/solidus/install/install_generator/install_frontend.rb

@@ -22,12 +22,14 @@ def call(frontend)
 
       private
 
-      def bundle_command(command, env = {})
+      def bundle_command(command)
         @generator_context.say_status :run, "bundle #{command}"
+        bundle_path = Gem.bin_path("bundler", "bundle")
+
         BundlerContext.bundle_cleanly do
           system(
             Gem.ruby,
-            Gem.bin_path("bundler", "bundle"),
+            bundle_path,
             *command.shellsplit,
           )
         end