Restructure CI workflow

[JLHwung]

Q	A
License	MIT

This PR restructured current CI workflow and move linux node unit tests to GitHub workflows.

In GitHub workflows we shared built babel artifacts across multiple unit test executors, thus we can reduce the overall build time in our CI. The new CI now always build Babel on latest node.js versions. If node.js latest incurs any regression, we can always pin the node version of the build step.

A comparison of CI time between main and this PR.

Item	main	This PR	CI Speed Improvments
Travis CI	13 min 21 sec	8 min 40 sec	54%
GitHub CI	8 min 20 sec	10 min 9 sec	-22%
max(Travis CI, GitHub CI)	13 min 21 sec	10 min 9 sec	30%

Data Source

main:
Travis CI: https://travis-ci.com/github/babel/babel/builds/181136937
GitHub CI: https://github.com/babel/babel/actions/runs/222502540

This PR:
Travis CI: https://travis-ci.com/github/JLHwung/babel/builds/181140151
GitHub CI: https://github.com/JLHwung/babel/actions/runs/222540002

This PR includes commits from #11781 , I will rebase later. You can review the actual changes on https://github.com/babel/babel/pull/12002/files/b9904078208917d338292ccad5ec544e7e53ae44..38d0ea99f83d5ec07b94a3bd0e2ddc2647c0bfdd

[babel-bot]

Build successful! You can test your changes in the REPL here: https://babeljs.io/repl/build/27934/

[codesandbox-ci]

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Latest deployment of this branch, based on commit f9c82f9:

Sandbox	Source
babel-repl-custom-plugin	Configuration
babel-plugin-multi-config	Configuration

[JLHwung]

This step is moved to test executor (instead of the Build job) because it is IO-intensive (1000 files), caching and restoring are slower than generating.