test: ensure no .env when EncryptionTest is run

[paulbalandan]

Description
For contributors, when running EncryptionTest and there is a .env file in the root, tests will fail because of a set encryption key.

There were 2 failures:

1) CodeIgniter\Encryption\EncryptionTest::testConstructor
Failed asserting that a string is empty.

/Users/paul/Workspace/CodeIgniter4/tests/system/Encryption/EncryptionTest.php:61

2) CodeIgniter\Encryption\EncryptionTest::testServiceWithoutKey
Failed asserting that exception of type "CodeIgniter\Encryption\Exceptions\EncryptionException" is thrown.

This PR will backup the .env and unset any environment variables already set by the file.

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide