Add environment variable for Julia CI job

Set JULIA_NUM_THREADS environment variable to '1'.

Author: ViralBShah

.github/workflows/CI.yml

@@ -9,6 +9,8 @@ jobs:
   test:
     name: Julia ${{ matrix.version }} - ${{ matrix.os }} - ${{ matrix.arch }} - ${{ github.event_name }}
     runs-on: ${{ matrix.os }}
+    env:
+      JULIA_NUM_THREADS: '1'
     strategy:
       fail-fast: false
       matrix: