[ruby-core:103132] [Ruby master Bug#17767] `Cloned ENV` inconsistently returns `ENV` or `self`

Issue #17767 has been reported by kachick (Kenichi Kamiya).

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Bug #17767: `Cloned ENV` inconsistently returns `ENV` or `self`
https://bugs.ruby-lang.org/issues/17767

* Author: kachick (Kenichi Kamiya)
* Status: Open
* Priority: Normal
* ruby -v: ruby 3.0.0p0 (2020-12-25 revision 95aff21468) [x86_64-darwin20]
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN, 3.0: UNKNOWN
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GH-PR: https://github.com/ruby/ruby/pull/4344

Is this an expected behavior?  

```console
$ ruby -v
ruby 3.0.0p0 (2020-12-25 revision 95aff21468) [x86_64-darwin20]
```

```ruby
cloned_env = ENV.clone

p ENV.each_key{}.equal?(ENV) #=> true
p cloned_env.each_key{}.equal?(cloned_env) #=> true

ENV.delete('TEST')

err = ENV.fetch('TEST') rescue $!
p err.receiver.equal?(ENV) #=> true
err = cloned_env.fetch('TEST') rescue $!
p err.receiver.equal?(cloned_env) #=> false

ENV['TEST'] = 'TRUE'
p ENV.select!{ false }.equal?(ENV) #=> true

cloned_env['TEST'] = 'TRUE'
p cloned_env.select!{ false }.equal?(cloned_env) #=> false
```

I guess ruby don't care `Cloned ENV` objects.
Yes, anyone basically do not write these code.
But the behaviors looks weird to me.  

Should block to clone?
Should return ENV?
Should return self?

I think `they return self` makes consistently behaviors and does not break compatibilities.
So I have created this PR  



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