Pass on USYM_UPLOAD_AUTH_TOKEN if defined (#234)

To automatically upload symbols to unity, we need to define the `USYM_UPLOAD_AUTH_TOKEN` variable. Currently the build container ignores this variable, even if it's defined in the github action.

```
2021-03-26T02:35:35.5938747Z time="2021-03-26T02:35:35Z" level=fatal msg="Please provide an auth token with USYM_UPLOAD_AUTH_TOKEN environment variable"
```

src/model/docker.ts

@@ -47,6 +47,7 @@ class Docker {
         --env UNITY_PASSWORD \
         --env UNITY_SERIAL \
         --env UNITY_VERSION="${version}" \
+        --env USYM_UPLOAD_AUTH_TOKEN \
         --env PROJECT_PATH="${projectPath}" \
         --env BUILD_TARGET="${platform}" \
         --env BUILD_NAME="${buildName}" \