feat(corp/ops): add NixOS profile for Yandex Cloud machines

Sets up a virtual machine image that is bootable on Yandex Cloud.

There are some slightly wonky behaviours still, like cloud-init
apparently putting all keys into root's authorized_keys no matter what
is specified in the metadata, but it does work now.

Change-Id: I57dcb7fcfa6872a28855dc1347f73a6db3c56828
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8496
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>

corp/ops/default.nix

@@ -34,4 +34,13 @@ depot.nix.readTree.drvTargets rec {
 
     yc.attr = "corp.ops.yc-cli";
   };
+
+  # Base image for Yandex VMs.
+  yandex-base-image = (depot.third_party.nixos {
+    configuration = { ... }: {
+      imports = [
+        (depot.path.origSrc + ("/corp/ops/modules/yandex-cloud.nix"))
+      ];
+    };
+  }).config.system.build.yandexCloudImage;
 }