Only test older Julia versions on ubuntu

Author: MilesCranmer

.github/workflows/CI.yml

@@ -27,13 +27,16 @@ jobs:
       fail-fast: false
       matrix:
         julia-version:
-          - '1.6.7'
-          - '1.7.2'
           - '1.8.3'
         os:
           - ubuntu-latest
           - windows-latest
           - macOS-latest
+        include:
+          - os: ubuntu-latest
+            julia-version: '1.7.2'
+          - os: ubuntu-latest
+            julia-version: '1.6.7'
 
     steps:
       - uses: actions/checkout@v2