Change nightly to v1.6 in CI

Author: odow

.github/workflows/ci.yml

@@ -23,7 +23,7 @@ jobs:
       fail-fast: false
       matrix:
         machine:
-          - version: 'nightly'
+          - version: '1.6'
             os: ubuntu-latest
             arch: x64
           - version: '1'