coveralls doesn't work from GitHub actions. Its "github" service type
uses the GITHUB_TOKEN from the action, which only has read access when
invoked from a forked repository.
codecov gets this right: it validates that an uploaded coverage file
originated from a GitHub action run by asking GitHub, and then uses its
OAuth credentials (through the Marketplace App) to comment and set
status.
Also, coveralls requires a third-party package to convert go's coverage
report to a format it understands. codecov detects the format
server-side. It also handles go's coverage format better: it highlights
lines with "some coverage" in yellow (coveralls has no concept of this).
