test(@nestjs/config) reset env correctly

[bejewel-kyoungmin]

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• The commit message follows our guidelines: https://github.com/nestjs/nest/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
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PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Other... Please describe:

What is the current behavior?

Issue Number: N/A

I made a similar test file in config repo simply.

describe('Test Case', () => {
  let envBackup;
  beforeAll(() => {
    envBackup = process.env;
  });

  describe('Test Case 1', () => {
    it('run test 1', () => {
      process.env['a'] = "8000";
      expect(process.env.a).toBe("8000");
    });
  });

  describe('Test Case 2', () => {
    it('run test 2', () => {
      expect(process.env.a).toBe("8000");
    });
  });

  afterEach(() => {
    process.env = envBackup;
  });
});

In this case we expect run test 2 to be failed. We guess the process.env will be reset in afterEach.
but it succeed.

Because envBackup variable is reference of process.env. If you change either envBackup or process.env, both of them will be updated. It doesn't be reset.
The first test case affects second test case. in second test case process.env.a is not undefined.

What is the new behavior?

Fortunately test cases in config repo have developed independently. All test cases pass.
But I wanna remove a potential risk.
I changed they don't refer each other.
And I hope to add more test cases after merging. : )

Does this PR introduce a breaking change?

• Yes
• No

Other information

[kamilmysliwiec]

lgtm