Always install go and vault: the check may accidentally use the CircleCI
pre-installed go version instead of the cached version.
Also, always install from sources without caching. Go/vault
installation is basically a tarball download. That's equivalent to
CirleCI caching without the complexity. These steps don't add much time
either.
Lastly, infer the OS when downloading tarball to avoid managing them in
the job spec.
This adopts pattern used by Vault, where we split CircleCI yaml config
into multiple files that get packed and translated to 2.0.
This has two motivations: First, to ease translating config to CircleCI
2.0 so it can run on Enterprise private repository. Second and most
importantly, it also adding Enterprise specific jobs in separate files
with reduced config file merging conflict resolution.
