hydra-queue-runner: Get store mode configuration from hydra.conf

To use the local Nix store (default):

  store_mode = direct

To use a local binary cache:

  store_mode = local-binary-cache
  binary_cache_dir = /var/lib/hydra/binary-cache

To use an S3 bucket:

  store_mode = s3-binary-cache
  binary_cache_s3_bucket = my-nix-bucket

Also, respect binary_cache_{secret,public}_key_file for signing the
binary cache.

src/hydra-queue-runner/state.hh

@@ -262,6 +262,8 @@ private:
 
     nix::Path hydraData, logDir;
 
+    std::map<std::string, std::string> hydraConfig;
+
     /* The queued builds. */
     typedef std::map<BuildID, Build::ptr> Builds;
     Sync<Builds> builds;