Coveralls upload is failing

[mathjazz]

Checks are often failing at the "Coveralls upload" step: https://github.com/projectfluent/fluent-rs/runs/3186541921 https://github.com/projectfluent/fluent-rs/runs/3124033996 https://github.com/projectfluent/fluent-rs/runs/3019102558

The failure seems unrelated to the patch.

Here's the error output:

Run coverallsapp/github-action@master
  with:
    github-token: ***
    path-to-lcov: coverage.lcov
    coveralls-endpoint: https://coveralls.io
Using lcov file: coverage.lcov
Error:  "2021-07-28T22:16:14.684Z"  'error from lcovParse: ' 'Failed to parse string'
Error:  "2021-07-28T22:16:14.686Z"  'input: ' 'TN:\n'
Error:  "2021-07-28T22:16:14.686Z"  'error from convertLcovToCoveralls'
Error: Failed to parse string