Update dependency cross-env to v6

[renovate[bot]]

This PR contains the following updates:

Package	Type	Update	Change
cross-env	devDependencies	major	^5.2.0 -> ^6.0.0

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Release Notes

kentcdodds/cross-env

### [`v6.0.0`](https://togithub.com/kentcdodds/cross-env/releases/v6.0.0) [Compare Source](https://togithub.com/kentcdodds/cross-env/compare/v5.2.1...v6.0.0) ##### Bug Fixes - bumped cross-spawn to version 7.0.0 ([#​211](https://togithub.com/kentcdodds/cross-env/issues/211)) ([61ebf59](https://togithub.com/kentcdodds/cross-env/commit/61ebf59)) ##### BREAKING CHANGES - Drop support for Node.js < 7.

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