Downgrade execa to 1.0.0 (to actually support Node 8.0.0)

[jennifer-shehane]

• Closes Error when installing cypress using Node.js version < 8.12.0 #6512

User facing changelog

• We fixed an issue where Cypress would not install using Node.js 8 due to a dependency's code not being compatible with Node.js version lower than 8.12.0.
• Downgraded execa from 3.0.0 to 1.0.0.

Additional details

• execa 1.0.0 engines allows code to run in Node.js 8.0.0
• This supersedes Downgrade execa to 2.0.0 (which supports Node.js 8.0.0) #6513 (which only fixed the issue with node_engines and not the actual use of code that will not work in 8.0.0)
• Our tests against node versions was not properly failing after last PR: Tests against cypress-test-node-versions not properly failing in repo? #6559

How has the user experience changed?

Should no longer see the errors below during install:

yarn

error execa@3.3.0: The engine "node" is incompatible with this module. Expected version "^8.12.0 || >=9.7.0". Got "8.0.0"
error Found incompatible module.

npm install

> cypress@4.0.2 postinstall /Users/jennifer/Dev/cypress-transform-test/node_modules/cypress
> node index.js --exec install

/Users/jennifer/Dev/cypress-transform-test/node_modules/execa/index.js:18
	const env = extendEnv ? {...process.env, ...envOption} : envOption;
	                         ^^^

SyntaxError: Unexpected token ...
    at createScript (vm.js:74:10)
    at Object.runInThisContext (vm.js:116:10)
    at Module._compile (module.js:533:28)
    at Object.Module._extensions..js (module.js:580:10)
    at Module.load (module.js:503:32)
    at tryModuleLoad (module.js:466:12)
    at Function.Module._load (module.js:458:3)
    at Module.require (module.js:513:17)
    at require (internal/module.js:11:18)
    at Object.<anonymous> (/Users/jennifer/Dev/cypress-transform-test/node_modules/cypress/lib/util.js:18:13)

[cypress-bot]

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User Experience

• The feature/bugfix is self-documenting from within the product.
• The change provides the end user with a way to fix their problem (no dead ends).

Functionality

• The code works and performs its intended function with the correct logic.
• Performance has been factored in (for example, the code cleans up after itself to not cause memory leaks).
• The code guards against edge cases and invalid input and has tests to cover it.

Maintainability

• The code is readable (too many nested 'if's are a bad sign).
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Quality

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• There's not a module from the ecosystem that should be used instead.
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Internal

• The original issue has been tagged with a release in ZenHub.