Debug CI: Reduce nthreads

Author: tkf

.github/workflows/ci.yml

@@ -18,7 +18,8 @@ jobs:
         nthreads:
           - '1'
           - '2'
-          - '8'
+          - '3'
+          - '4'
       fail-fast: false
     name: Test Julia ${{ matrix.julia-version }} nthreads=${{ matrix.nthreads }}
     steps: