Update dependency execa to v4

Created by: renovate[bot]

This PR contains the following updates:

Package	Type	Update	New value	References	Sourcegraph
execa	devDependencies	major	^4.0.0	source

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Release Notes

sindresorhus/execa

v4.0.0

Compare Source

Breaking changes

• Require Node.js 10 (5a9c76f)
• Add stderr and stdout to error.message. A new property error.shortMessage is now available to retrieve the error message without stderr nor stdout (#​397)

Bug fixes

• Fix childProcess.kill() not working with Electron (#​400)

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