feat(ops/pipelines): Dynamically generate CI pipeline from targets

Create the pipeline by outputting a file that contains nix-build
invocations for each target's *derivation path*.

Each invocation has a generated Nix expression passed to it with `-E`
which fetches the correct target from the tree while correctly
handling targets with strange characters (such as in Go-packages).

This makes it possible to run target-level granular pipelines. We're
getting somewhere!

Change-Id: Ia6946e389dafd1d4926130bb8891446d6e17133b
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1855
Tested-by: BuildkiteCI
Reviewed-by: glittershark <grfn@gws.fyi>
Reviewed-by: lukegb <lukegb@tvl.fyi>

ops/nixos/default.nix

@@ -47,5 +47,5 @@ rec {
   #
   # TODO(tazjin): Refactor the whole systems setup, it's a bit
   # inconsistent at the moment.
-  whitbySystem = (nixosFor whitby).system // { __readTree = true; };
+  whitbySystem = (nixosFor whitby).system;
 }