[v15] Use default CDN Base URL by client tools managed updates for OSS build

[vapopov]

In this PR added fix discovered during the testing tsh managed update from v 15.4.29-30 which was affected by previous backport of #51934

Since v15 licensed under Apache, we should always use default CDN for client tools managed updates

Build-tag: https://github.com/gravitational/teleport.e/actions/runs/13934159552

Changelog: Fixed an issue where the default CDN Base URL for client tools managed updates was incorrectly required for OSS builds.

[vapopov]

Verified on dev build, update is triggered by login to v17 cluster:

[root@vbox ~]# tsh version
Teleport v15.4.30-dev.vapopov.3 git:v15.4.30-dev.vapopov.3-0-g546ff08 go1.23.7
[root@vbox ~]# tsh login --proxy vadym.teleport.sh:443 --user vadym.popov@goteleport.com

WARNING
Detected potentially incompatible client and server versions.
Minimum client version supported by the server is 16.0.0-aa but you are using 15.4.30-dev.vapopov.3.
Please upgrade tsh to 16.0.0-aa or newer or use the --skip-version-check flag to bypass this check.
Future versions of tsh will fail when incompatible versions are detected.

Update progress: [▒▒▒▒▒▒▒▒▒▒] (Ctrl-C to cancel update)
Enter password for Teleport user vadym.popov@goteleport.com: