ci : switch threads to 1 (#2138)

.github/workflows/build.yml

@@ -19,6 +19,7 @@ env:
   BRANCH_NAME: ${{ github.head_ref || github.ref_name }}
   GGML_NLOOP: 3
   GGML_NITER: 1
+  GGML_N_THREADS: 1
 
 jobs:
   ubuntu-focal-make: