bin_path_spec.rb relied to available Ruby environment with after `make install`

But we stub-out GEM_HOME variable for test-bundled-gems and others on ruby/ruby. It means
the installation path mismatched with GEM_HOME variable always. We can't test this example
collectly.

```
1)
Gem.bin_path finds executables of default gems, which are the only files shipped for default gems FAILED
Expected File.exist? "/Users/hsbt/Documents/github.com/ruby/ruby/.bundle/gems/bundler-2.7.0.dev/exe/bundle"
to be truthy but was false
```

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