CI Support: SemaphoreCI

[drazisil-codecov]

I never expected the Git SHA's from SemaphoreCI to be different
than the PR. The issue was the uploader using the SEMAPHORE_GIT_SHA
(which is a non-existent SHA as you pointed out) instead of
SEMAPHORE_GIT_PR_SHA. So I resolved the issue by forcing the uploader
to use the "-C" CLI arg.

This can be done by following the directions here https://github.com/codecov/uploader/blob/24772d18bd16800d9f57ed3a01c29b6f21a2317a/CONTRIBUTING.md#adding-a-new-ci-provider

This is already supported in the API https://github.com/codecov/codecov-api/blob/4c595ac7c74d496ac5f6fd440a93a00c6546cb1f/upload/constants.py#L142-L147 so no changes where be needed there. The "service" query value will need to match semaphore