[ ci ] Run tests in 2 threads

Author: spcfox

.github/workflows/ci-deptycheck.yml

@@ -206,6 +206,8 @@ jobs:
       - get-test-sets
     runs-on: ubuntu-latest
     container: ghcr.io/stefan-hoeck/idris2-pack:latest
+    env:
+      NUM_THREADS: 2
     strategy:
       fail-fast: false # all test cases are more or less independent
       matrix: