Coveralls stopped working back in July when we reached build number 3790
because back when we used Travis we created builds from numbers 3791 to
35700.
After trying to change the numbers in several ways, all of them
resulting in a "No build matching CI build number" message, we're trying
the Github Action provided by Coveralls instead. In order to make it
work, we need to add the `simplecov-lcov` gem and generate the report at
`coverage/lcov.info`. Note that, for some reason, `simplecov-lcov`
doesn't seem to depend on `simplecov` and we need `simplecov` 0.18 or
later, so we're manually adding this dependency as well.
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