hydra-eval-jobset: Use nix-eval-jobs instead of hydra-eval-jobs

incrementally ingest eval results

nix-eval-jobs streams output, unlike hydra-eval-jobs. Now that we've
migrated, we can use this to:

1. Use less RAM by avoiding buffering a whole eval's worth of metadata
   into a Perl string and an array of JSON objects.
2. Make evals latency a bit lower by allowing the queue runner to start
   ingesting builds faster.

Also use the newly-restored constituents support in `nix-eval-jobs`

Note, we pass --workers and --max-memory-size to n-e-j

Lost in the h-e-j -> n-e-j migration, causing evaluation to always be
single threaded and limited to 4GiB RAM. Follow the config settings like
h-e-j used to do (via C++ code).

`nix-eval-jobs` should check `hydraJobs` and then `checks` with flakes

(cherry picked from commit 6d4ccff43c41adaf6e4b2b9bced7243bc2f6e97b)
(cherry picked from commit b0e9b4b2f99f9d8f5c4e780e89f955c394b5ced4)
(cherry picked from commit cdfc5c81e8037d3e4818a3e459d0804b2c157ea9)
(cherry picked from commit 4b107e6ff36bd89958fba36e0fe0340903e7cd13)

Co-Authored-By: Maximilian Bosch <maximilian@mbosch.me>

t/jobs/meta.nix

@@ -0,0 +1,17 @@
+with import ./config.nix;
+{
+  full-of-meta =
+    mkDerivation {
+      name = "full-of-meta";
+      builder = ./empty-dir-builder.sh;
+
+      meta = {
+        description = "This is the description of the job.";
+        license = [ { shortName = "MIT"; } "BSD" ];
+        homepage = "https://example.com/";
+        maintainers = [ "alice@example.com" { email = "bob@not.found"; } ];
+
+        outPath = "${placeholder "out"}";
+      };
+    };
+}