Fix CI versions (#970)

Author: tkf

.github/workflows/aot.yml

@@ -19,7 +19,7 @@ jobs:
         python-version: ['3.x']
         julia-version:
           - '1'
-          - '~1.7.0-rc1'
+          - '1.6'
           # - 'nightly'  # TODO: reenable
       fail-fast: false
     env:

.github/workflows/system.yml

@@ -48,7 +48,7 @@ jobs:
             architecture: x64
             python-version: '2.7'
             julia-version: '1'
-          - os: windows-latest
+          - os: windows-2019
             architecture: x64
             python-version: '2.7'
             julia-version: '1'