Improve #6041, Migrate to CircleCI 2.0

[serima]

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• It's submitted to the dev branch for v2.x (or to a previous version branch), not the master branch
• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)
• All tests are passing: https://github.com/vuejs/vue/blob/dev/.github/CONTRIBUTING.md#development-setup
• New/updated tests are included

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)

Other information:

We are going to migrate to CircleCI 2.0 with #6041, but with a simpler description, we can make the build run time shorter.

When caching node_modules, the build is now finished in 1 minute 20 seconds.
https://circleci.com/workflow-run/7ea0d4e0-48ca-449d-9a52-9880dbbd93a5

In the case of 2.0, we have to describe all of the procedures procedurally, but on the contrary, I think that approach is easy to understand.

Please check this PR 😃 @yyx990803 @posva

[yyx990803]

Very nice! I noticed that our CI actually has a 4x parallelism limit, so I merged some of the jobs together to minimize queueing.

[serima]

@yyx990803 Thank you for balancing the test! It got faster!