ci: Update CircleCI pipeline to run Codecov

[thomasrockhu-codecov]

This duplicates https://github.com/vuejs/vue/pull/12402 due to a branch deletion mistake.

Tom from Codecov here. The original intent of the build was to only run Codecov on non-PRs. But it seems like every commit has an associated PR now. The Codecov page shows no commits for the past year, and viewing any build of a commit in that timeframe (e.g. 6e22f770) will show that CIRCLE_PULL_REQUEST and CI_PULL_REQUEST are set.

In any case, no coverage report has been uploaded in quite some time. I realize that this will change the PR experience for users, so happy to change this to whatever makes the most sense for Vue.

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

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

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

• [ ] Yes
• [x] No

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

The PR fulfills these requirements:

• [x] 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)
• [x] 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: The npm version of the uploader is being deprecated in favor of the new universal uploader. Also, CI_PULL_REQUEST has been deprecated in favor of CIRCLE_PULL_REQUEST.

[thomasrockhu-codecov]

fyi @posva

[thomasrockhu]

gentle bump @posva

[thomasrockhu]

@posva is this pending any other changes?