[Fix #10788] Relax Style/FetchEnvVar to allow ENV[] in LHS of ||

[j-miyake]

Fixes #10788

This PR relaxes Style/FetchEnvVar cop to allow ENV[] in LHS of ||.
ENV[] in LHS of || operator can be allowed by this cop because the || is put by the developer who knows the LHS may be nil.

Current behavior

ENV['x'] || y
ENV['y'] || ENV['y'] || z

# corrected into..

ENV.fetch('x'){y}
ENV.fetch('x'){ENV.fetch('y'){z}}

New behavior

ENV['x'] || y
ENV['y'] || ENV['y'] || z

# no offenses

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• Squashed related commits together.
• Added tests.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
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[bbatsov]

Looks good. Thanks!