feat(nix/buildkite): support meta.timeout

This uses the nixpkgs convention of meta.timeout, and adds a
timeout_in_minutes field to the pipeline step in case its set.

Fixes https://b.tvl.fyi/issues/285.

Change-Id: Ia72e3832f14bf9172319bce070c5b0944f1c96fe
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8970
Autosubmit: flokli <flokli@flokli.de>
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

nix/buildkite/default.nix

@@ -84,6 +84,8 @@ rec {
       # always runs. This allows build steps uploaded in batches to
       # start running before all batches have been uploaded.
       depends_on = ":init:";
+    } // lib.optionalAttrs (target ? meta.timeout) {
+      timeout_in_minutes = target.meta.timeout / 60;
     };
 
   # Helper function to inelegantly divide a list into chunks of at