chore(ci): remove unused and undeclared OS matrix env

Author: heitorlessa

.github/workflows/python_build.yml

@@ -28,7 +28,6 @@ jobs:
       matrix:
         python-version: [3.7, 3.8, 3.9]
     env:
-      OS: ${{ matrix.os }}
       PYTHON: ${{ matrix.python-version }}
     steps:
       - uses: actions/checkout@v3
@@ -56,6 +55,6 @@ jobs:
         with:
           file: ./coverage.xml
           # flags: unittests
-          env_vars: OS,PYTHON
+          env_vars: PYTHON
           name: aws-lambda-powertools-python-codecov
           # fail_ci_if_error: true  # failing more consistently making CI unreliable despite all tests above passing