chore: automatically acquire branch name for CircleCI

[quantizor]

https://circleci.com/docs/variables/

Since Codecov provides a CircleCI orb, let's make it easier to acquire the branch name without additional config.

[quantizor]

@thomasrockhu-codecov @matt-codecov when you get a moment, thanks!

[matt-codecov]

@quantizor thanks for the contribution!

i'm a little perplexed because we supposedly already support this:

• we have a CI adapter for CircleCI that exposes the branch name with that same CIRCLE_BRANCH env var
• we detect whether the current environment is CircleCI with the CI and CIRCLECI env vars

do you have an example CI run where codecov was failing to get the circleci branch?