feat(workflow): document SENTRY_MYPY_PRE_PUSH env var

src/docs/workflow.mdx

@@ -32,3 +32,5 @@ We're slowly working to add typing to our Python codebase, using `mypy` to check
 - Check that the file you're modifying is not on the exclude list from `pyproject.toml`. In other words, you need to opt-in the file for type checking to happen.
 
 - To test types locally, you can run `mypy path/to/file` or just `mypy` to run against all files.
+
+- To automatically run `mypy` during pre-push, export the `SENTRY_MYPY_PRE_PUSH` environment variable set to `1`.