feat(server): Add configuration option for Nix build timeouts

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

tools/nixery/server/builder/builder.go

@@ -119,6 +119,7 @@ func BuildImage(ctx *context.Context, cfg *config.Config, cache *BuildCache, ima
 		}
 
 		args := []string{
+			"--timeout", cfg.Timeout,
 			"--argstr", "name", image.Name,
 			"--argstr", "packages", string(packages),
 		}