docs: Use correct link to build badge image and build status page. (#1390)

I'm not sure what happened, but the old image url doesn't work anymore.

Also links to the canonical build status page; the old postsubmit url
simply
redirects to the canonical url.

Author: rickeylev

README.md

@@ -1,6 +1,6 @@
 # Python Rules for Bazel
 
-[![Build status](https://badge.buildkite.com/1bcfe58b6f5741aacb09b12485969ba7a1205955a45b53e854.svg?branch=main)](https://buildkite.com/bazel/python-rules-python-postsubmit)
+[![Build status](https://badge.buildkite.com/0bcfe58b6f5741aacb09b12485969ba7a1205955a45b53e854.svg?branch=main)](https://buildkite.com/bazel/rules-python-python)
 
 ## Overview