outposts: fix flow executor when using subpath

[ikob]

Details

The API URL now includes the web path. Outposts can coexist with authentik deployed behind a reverse proxy under a web path. This PR fix outposts/radius as well.

Note: The AUTHENTIK_HOST environment variable should include the web path, e.g., http://authentik.io:9000/authentik/.

closes #13674 (Address comment raised by @najose)

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Checklist

• Local tests pass (ak test authentik/)
• The code has been formatted (make lint-fix)

If an API change has been made

• The API schema has been updated (make gen-build)

If changes to the frontend have been made

• The code has been formatted (make web)

If applicable

• The documentation has been updated
• The documentation has been formatted (make docs)

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