impure-checks: limit number of workers to number of tests

checks/impure/flake-module.nix

@@ -19,6 +19,7 @@
             [
               pkgs.gitMinimal
               pkgs.nix
+              pkgs.coreutils
               pkgs.rsync # needed to have rsync installed on the dummy ssh server
             ]
             ++ self'.packages.clan-cli-full.runtimeDependencies
@@ -30,7 +31,12 @@
         # this disables dynamic dependency loading in clan-cli
         export CLAN_NO_DYNAMIC_DEPS=1
 
-        nix develop "$ROOT#clan-cli" -c bash -c "TMPDIR=/tmp python -m pytest -m impure ./clan_cli $@"
+        jobs=$(nproc)
+        # Spawning worker in pytest is relatively slow, so we limit the number of jobs to 13
+        # (current number of impure tests)
+        jobs="$((jobs > 13 ? 13 : jobs))"
+
+        nix develop "$ROOT#clan-cli" -c bash -c "TMPDIR=/tmp python -m pytest -n $jobs -m impure ./clan_cli $@"
       '';
     };
 }