assumption for CODEBUILD_SOURCE_VERSION is too strict

[dbollinger]

The codebuild update here assumes CODEBUILD_SOURCE_VERSION is always the PR number: https://github.com/codecov/codecov-node/pull/135/files#diff-f9543833f7107b39b6ce133555c04e02R27

However, it can be a number of other options depending on build setup:

For GitHub, GitHub Enterprise, and Bitbucket it is the commit ID, branch name, or tag name associated with the version of the source code to be built. Note For a GitHub or GitHub Enterprise build that is triggered by a webhook pull request event, it is pr/pull-request-number.

https://docs.aws.amazon.com/codebuild/latest/userguide/build-env-ref-env-vars.html