Clean go module cache after test

Workaround following error

```
1) bundle gem gem naming with underscore --ext parameter set with go includes go_gem extension in extconf.rb
   Failure/Error: FileUtils.rm_r(dir)

   Errno::EACCES:
     Permission denied @ apply2files - /home/runner/work/rubygems/rubygems/bundler/tmp/2.2/home/go/pkg/mod/gopkg.in/[email protected]/decode_test.go
   # ./spec/support/helpers.rb:37:in `block in reset!'
   # ./spec/support/helpers.rb:21:in `each'
   # ./spec/support/helpers.rb:21:in `reset!'
   # ./spec/spec_helper.rb:130:in `block (2 levels) in <top (required)>'
   # /home/runner/work/rubygems/rubygems/lib/rubygems.rb:303:in `load'
   # /home/runner/work/rubygems/rubygems/lib/rubygems.rb:303:in `activate_and_load_bin_path'
```

Files installed with `go get` have permissions set to 444
ref. golang/go#35615

```
$ ls -l /home/runner/work/rubygems/rubygems/bundler/tmp/2.2/home/go/pkg/mod/gopkg.in/[email protected]/decode_test.go
-r--r--r-- 1 runner runner 42320 Nov 15 06:38 /home/runner/work/rubygems/rubygems/bundler/tmp/2.2/home/go/pkg/mod/gopkg.in/[email protected]/decode_test.go
```

So they cannot be deleted by `FileUtils.rm_r`.
Therefore, this is necessary to execute `go clean -modcache` separately from `FileUtils.rm_r` to circumvent it.

Author: sue445

bundler/spec/commands/newgem_spec.rb

@@ -1763,6 +1763,10 @@ def create_temporary_dir(dir)
         bundle ["gem", gem_name, flags].compact.join(" ")
       end
 
+      after do
+        sys_exec("go clean -modcache", raise_on_error: true) if installed_go?
+      end
+
       it "is not deprecated" do
         expect(err).not_to include "[DEPRECATED] Option `--ext` without explicit value is deprecated."
       end