feat(ops/buildkite): Bootstrap Buildkite Terraform configuration

In order to run this the secrets needs to be sourced, e.g.:

  eval $(age --decrypt -i ~/.ssh/id_ed25519 $(git rev-parse --show-toplevel)/ops/secrets/tf-buildkite.age)

Change-Id: I9f6a02c0dac22f584181635861ddbb06cf849f14
Reviewed-on: https://cl.tvl.fyi/c/depot/+/5838
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>
Reviewed-by: tazjin <tazjin@tvl.su>

ops/buildkite/default.nix

@@ -0,0 +1,7 @@
+{ depot, pkgs, ... }:
+
+depot.nix.readTree.drvTargets {
+  terraform = pkgs.terraform.withPlugins (p: [
+    p.buildkite
+  ]);
+}