Provide prebuilt binaries for FreeBSD #58

This issue is basically codecov/uploader#849 for the modern era. 😄 The move from the Node.js-based uploader to the Python-based CLI means that coverage can now be submitted from FreeBSD, e.g. on Cirrus CI and Travis, which is awesome! However, since prebuilt binaries are not available for FreeBSD like they are for other platforms, FreeBSD CI builds need to install from PyPI via pip. That in itself is not a problem—however, there is no Python wheel platform tag for FreeBSD, so installation requires a full source build of the package and all of its dependencies. This takes about 7 minutes of CI time on Cirrus for every run. In contrast, downloading and using the official prebuilt binaries from https://cli.codecov.io takes just a few seconds. It would be great to be able to avoid using all of that extra compute time.

Just as some of the binaries here are built using QEMU on GitHub Actions, FreeBSD binaries could be built using https://github.com/vmactions/freebsd-vm. (Alternatively, binaries for AArch64 Linux, both glibc and musl, could be built natively on Cirrus, which also natively supports FreeBSD, but I imagine y'all don't want to maintain a separate CI setup alongside GitHub Actions.)

Admittedly I don't know much about Python packaging but I'm more than happy to help however I can! cc also @lwhsu, who also offered to support for the previous issue.

codecov/codecov-cli#438