ci: explicitly test for 1.2.0

Author: mkniewallner

.github/workflows/installer.yml

@@ -31,13 +31,14 @@ jobs:
           - "--preview"
           - "--git https://github.com/python-poetry/poetry.git"
           - "--version 1.1.15"
+          - "--version 1.2.0"
         include:
           - os: Ubuntu
-            image: ubuntu-latest
+            image: ubuntu-22.04
           - os: Windows
             image: windows-2022
           - os: macOS
-            image: macos-11
+            image: macos-12
     defaults:
       run:
         shell: bash