The new build process can now call out to Nix to create layers and
upload them to the bucket if necessary.
The layer cache is populated, but not yet used.
Introduces three new types representing each of the possible package
sources and moves the logic for specifying the package source to the
server.
Concrete changes:
* Determining whether a specified git reference is a commit vs. a
branch/tag is now done in the server, and is done more precisely by
using a regular expression.
* Package sources now have a new `CacheKey` function which can be used
to retrieve a key under which a build manifest can be cached *if*
the package source is not a moving target (i.e. a full git commit
hash of either nixpkgs or a private repository).
This function is not yet used.
* Users *must* now specify a package source, Nixery no longer defaults
to anything and will fail to launch if no source is configured.
Adds a NIX_TIMEOUT environment variable which can be set to a number
of seconds that is the maximum allowed time each Nix builder can run.
By default this is set to 60 seconds, which should be plenty for most
use-cases as Nixery is not expected to be performing builds of
uncached binaries in most production cases.
Currently the errors Nix throws on a build timeout are not separated
from other types of errors, meaning that users will see a generic 500
server error in case of a timeout.
This fixes#47
