feat: add passing of Renovate environment variables

Inputs may now be provided using environment variables, as well as the existing inputs. None of the
inputs are required any more, so it is possible to use only environment variables. Nevertheless all
inputs must be provided in some way, either using the input or their corresponding environment
variables.

BREAKING CHANGE: The `configurationFile` input no longer has a default value. This means that a
value for it is now required using the `configurationFile` input or the `RENOVATE_CONFIG_FILE`
environment variable.

Closes #136.

.github/workflows/build.yml

@@ -48,6 +48,8 @@ jobs:
         run: npm run build
       - name: Renovate test
         uses: ./
+        env:
+          LOG_LEVEL: debug
         with:
           configurationFile: ${{ matrix.configurationFile }}
           token: ${{ secrets.RENOVATE_TOKEN }}