Use poetry 1.2.2

AFAICS from https://github.com/python-poetry/poetry/releases this is
backwards-compatible with 1.2.x. Crucially, it's also
forwards-compatible with 1.3.x's new lockfile format (see
python-poetry/poetry#6608)

Fixes #12.

action.yml

@@ -8,7 +8,7 @@ inputs:
   poetry-version:
     description: Poetry version to install via pip.
     required: false
-    default: "1.2.0"
+    default: "1.2.2"
   extras:
     description: >
       If present, a space separated list of extras to pass to