Expose kcov run results as HTML

[sandreim]

At this time coverage failures investigation require manually copying the files locally. This would reduce the time required to investigate a coverage failure and would easily highlight the code that requires testing.

Options that I can think of right now are:

• zip the output and upload it and provide URL for download
• upload and provide URL.

[serban300]

We're continuing to track this as a nice to have.

[dianpopa]

After discussions, we decided to make use of buildkite's artifact capability to store the zipped HTML results that kcov leaves behind after a coverage run.

[roypat]

We have switching from kcov to using grcov, and decided that instead of having our CI generate html artifacts and store them, we use codecov.io for coverage visualization: https://app.codecov.io/gh/firecracker-microvm/firecracker