Fix jinja search path for local file_client

Now that FS backends other than roots work with a local file_client,
there is no need to look only in the file_roots directories.

Fixes #17963.
Fix jinja tests

These tests were using the now-outdated method of looking in the
file_roots for cached templates in masterless mode.
Merge pull request #18860 from terminalmage/issue17963

Fix jinja search path for local file_client