Related to #6364 and #6363, this:
- Adds a new Docker image, `fleetdm/fleetctl` equipped with all necessary dependencies to build Fleet-osquery binaries for all platforms
- Modifies the package generation logic to special case this scenario via an environment variable `FLEETCTL_NATIVE_TOOLING`
- Adds a new GitHub workflow to test this
There are more details in the README, but part of the special-casing logic is in place to output the binaries to a folder named `build` when they are run with `FLEETCTL_NATIVE_TOOLING`, this is so we can persist the binary generated by the docker container via a bind mount:
```bash
docker run -v "$(pwd):/build" fleetdm/fleetctl package --type=msi
```
To test this changeset, I have generated packages for all platforms, both via the new Docker image and via the classic `fleetctl package`.
