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>

users/glittershark/system/home/default.nix

@@ -20,7 +20,7 @@ rec {
 
       lib.depot = depot;
     };
-  }) // { __readTree = true; };
+  });
 
   chupacabra = home ./machines/chupacabra.nix;
 }

users/glittershark/system/system/default.nix

@@ -5,7 +5,7 @@ rec {
 
   chupacabraSystem = (pkgs.nixos {
     configuration = chupacabra;
-  }).system // { __readTree = true; };
+  }).system;
 
   rebuilder =
     let

users/tazjin/nixos/default.nix

@@ -8,7 +8,7 @@ let
     configuration = lib.fix(config:
       foldl' lib.recursiveUpdate {} (map (c: c config) configs)
     );
-  }).system // { __readTree = true; };
+  }).system;
 
   caseFor = hostname: ''
     ${hostname})