Make the Bazel CI workflow use the local Python environment.

This is setting up to run mypy as per #496.

Change-Id: I48e67387313ac01d493086be8481b34f26bafe82
Reviewed-on: https://code-review.googlesource.com/c/re2/+/63230
Reviewed-by: Alex Chernyakhovsky <achernya@google.com>
Reviewed-by: Paul Wankadia <junyer@google.com>