feat(3p/nix/nix-daemon): Implement Worker::BuildDerivation handler

Implement the proto handler on the server side for
Worker::BuildDerivation. This includes several additions to the proto
which I had missed on the first pass, including the actual proto
definition for the Derivation itself and a few sequence number
reorderings which are fine because this is all provisional and not
deployed yet.

A couple things to note

- I implemented a couple constructors for nix classes that initialize
  themselves based on their proto variants, which felt nice and didn't
  end up causing any issues.
- I've made the conversions between the enum types in nix and in proto
  explicit via switch statements rather than using a static_cast, out of
  an abundance of caution that the error would get mismatched in the
  future and we'd convert the wrong thing to the wrong thing - this is
  verbose, but exceptionally future proof.

Change-Id: Iecf6b88e76bc37e49efa05fd65d6cd0cb0deffed
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1249
Tested-by: BuildkiteCI
Reviewed-by: tazjin <mail@tazj.in>
Reviewed-by: Kane York <rikingcoding@gmail.com>

third_party/nix/src/libstore/store-api.cc

@@ -18,6 +18,50 @@
 
 namespace nix {
 
+std::optional<BuildMode> build_mode_from(nix::proto::BuildMode mode) {
+  switch (mode) {
+    case nix::proto::BuildMode::Normal:
+      return BuildMode::bmNormal;
+    case nix::proto::BuildMode::Repair:
+      return BuildMode::bmRepair;
+    case nix::proto::BuildMode::Check:
+      return BuildMode::bmCheck;
+    default:
+      return {};
+  }
+}
+
+nix::proto::BuildStatus BuildResult::status_to_proto() {
+  switch (status) {
+    case BuildResult::Status::Built:
+      return proto::BuildStatus::Built;
+    case BuildResult::Status::Substituted:
+      return proto::BuildStatus::Substituted;
+    case BuildResult::Status::AlreadyValid:
+      return proto::BuildStatus::AlreadyValid;
+    case BuildResult::Status::PermanentFailure:
+      return proto::BuildStatus::PermanentFailure;
+    case BuildResult::Status::InputRejected:
+      return proto::BuildStatus::InputRejected;
+    case BuildResult::Status::OutputRejected:
+      return proto::BuildStatus::OutputRejected;
+    case BuildResult::Status::TransientFailure:
+      return proto::BuildStatus::TransientFailure;
+    case BuildResult::Status::CachedFailure:
+      return proto::BuildStatus::CachedFailure;
+    case BuildResult::Status::TimedOut:
+      return proto::BuildStatus::TimedOut;
+    case BuildResult::Status::MiscFailure:
+      return proto::BuildStatus::MiscFailure;
+    case BuildResult::Status::DependencyFailed:
+      return proto::BuildStatus::DependencyFailed;
+    case BuildResult::Status::LogLimitExceeded:
+      return proto::BuildStatus::LogLimitExceeded;
+    case BuildResult::Status::NotDeterministic:
+      return proto::BuildStatus::NotDeterministic;
+  }
+}
+
 bool Store::isInStore(const Path& path) const {
   return isInDir(path, storeDir);
 }