ci/eval: use sane defaults

Using these defaults makes it possible to just run `nix-build ci -A
eval.singleSystem` without passing additional arguments and get a sane
result back. Especially helpful when testing or debugging.

A `chunkSize` of 5000 is conservative to be able to run on systems with
less memory as well. Run-time is not impacted by that, as recent
benchmarks show.

(cherry picked from commit 75f40a150c)

ci/eval/default.nix

@@ -72,11 +72,11 @@ let
       # The system to evaluate.
       # Note that this is intentionally not called `system`,
       # because `--argstr system` would only be passed to the ci/default.nix file!
-      evalSystem,
+      evalSystem ? builtins.currentSystem,
       # The path to the `paths.json` file from `attrpathsSuperset`
       attrpathFile ? "${attrpathsSuperset { inherit evalSystem; }}/paths.json",
       # The number of attributes per chunk, see ./README.md for more info.
-      chunkSize,
+      chunkSize ? 5000,
       checkMeta ? true,
 
       # Don't try to eval packages marked as broken.