As part of the tests run in the CircleCI `aio_monitoring` job, we need
to retrieve `sitemap.xml` from the site under test. Previously, the URL
used to retrieve that contained a double-slash (`//`). At some point,
Firebase (which is used for hosting the site) stopped normalizing
double-slashes to a single slash, causing the test to fail.
This commit fixes the problem by ensuring that the constructed URLs do
not contain double-slashes.
PR Close#25641