feat(server): Reimplement creation & uploading of layers

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.

tools/nixery/server/layers/grouping.go

@@ -328,7 +328,7 @@ func dominate(budget int, graph *simple.DirectedGraph) []Layer {
 // GroupLayers applies the algorithm described above the its input and returns a
 // list of layers, each consisting of a list of Nix store paths that it should
 // contain.
-func GroupLayers(refs *RuntimeGraph, pop *Popularity, budget int) []Layer {
+func Group(refs *RuntimeGraph, pop *Popularity, budget int) []Layer {
 	graph := buildGraph(refs, pop)
 	return dominate(budget, graph)
 }