feat(nix/buildkite): add meta.ci.buildkiteExtraDeps

This allows setting a buildkiteExtraDeps in `meta.ci`, which will get
added to `depends_on` (which can also be a list).

It allows explicitly describing a dependency on another buildkite step,
either generated by nix/buildkite, or part of the static pipeline.

At some point we might want to expose our key calculation function too,
similar to how readTree exposes mkLabel already, but that's left for
a followup.

Change-Id: I793170401ccd3907ad8bf232b80ca7a492e1c942
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8980
Reviewed-by: tazjin <tazjin@tvl.su>
Autosubmit: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI

nix/buildkite/default.nix

@@ -83,7 +83,7 @@ rec {
       # Add a dependency on the initial static pipeline step which
       # always runs. This allows build steps uploaded in batches to
       # start running before all batches have been uploaded.
-      depends_on = ":init:";
+      depends_on = [ ":init:" ] ++ lib.optionals (target ? meta.ci.buildkiteExtraDeps) target.meta.ci.buildkiteExtraDeps;
     } // lib.optionalAttrs (target ? meta.timeout) {
       timeout_in_minutes = target.meta.timeout / 60;
     };