feat(ci): Add subtarget support for builds

We have naturally evolved a distinction between logical and physical
targets.

Physical targets are those which correspond directly to a tree
location on disk and can be built with `-A path.to.files`, while
logical targets are those that are exported from within an expression
but do not have a corresponding file on disk.

This change adds support for exporting logical targets from any tree
location by adding a `meta.targets` attribute containing keys into
itself, which will be consumed by the CI target gathering logic and
included in the generated pipeline.

Note that the labels for subtargets are syntactically different to
emphasise that they do not correspond to a file location. For example,
this change enables 'ops.nixos.whitbySystem' as a subtarget, which is
labeled in CI as `ops/nixos:whitbySystem`.

Change-Id: Ied09647a62c2ba98e3914548e3742ad422c63ecf
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1893
Tested-by: BuildkiteCI
Reviewed-by: glittershark <grfn@gws.fyi>

ops/nixos/default.nix

@@ -48,4 +48,5 @@ rec {
   # TODO(tazjin): Refactor the whole systems setup, it's a bit
   # inconsistent at the moment.
   whitbySystem = (nixosFor whitby).system;
+  meta.targets = [ "whitbySystem" ];
 }