Install codecov.io uploader into container

[roypat]

Needed for rust-vmm/community#183, to replace out homegrown, file-based coverage checks with codecov.io.

Summary of the PR

Please summarize here why the changes in this PR are needed.

Requirements

Before submitting your PR, please make sure you addressed the following
requirements:

• All commits in this PR have Signed-Off-By trailers (with
  git commit -s), and the commit message has max 60 characters for the
  summary and max 75 characters for each description line.
• All added/changed functionality has a corresponding unit/integration
  test.
• All added/changed public-facing functionality has entries in the "Upcoming
  Release" section of CHANGELOG.md (if no such section exists, please create one).
• Any newly added unsafe code is properly documented.

[RuoqingHe]

Looks good to me, thanks @roypat! I will work out codecov on RISC-V and come back later, let's get the wheels turning first 😉

[roypat]

Looks good to me, thanks @roypat! I will work out codecov on RISC-V and come back later, let's get the wheels turning first 😉

Thanks! I had a cursory glance around the codecov website, but it seems like they don't have a native riscv uploader :/