Add missing API root for external logs requests

[AlanGreene]

Changes

Resolves #3606
Replaces #3607

Somehow the external logs APIs in the client never used the API root when constructing the URL. This means that if the Dashboard is deployed at a non-root path, the requests never made it to the logs proxy.

Update the external logs URL util to include the API root in the same manner as we already do for all other APIs.

/kind bug

Submitter Checklist

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Release Notes

NONE

[AlanGreene]

Unrelated to this change, will follow-up separately
/test pull-tekton-dashboard-integration-tests

[briangleeson]

/lgtm

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