swiftly install 6.4-snapshot fails because url schema change

This is expected to work: ``` ktoso@cherudim [SIGINT] [11:43:48] [~/code/swiftly] [wip-handle-6.4 *] -> % swiftly install 6.4-snapshot Fetching the latest 6.4 development branch snapshot... ----------------------------- A new release of swiftly is available. Please run `swiftly self-update` to update. ----------------------------- Error: Unexpected response, expected status code: ok, response: undocumented(statusCode: 404, OpenAPIRuntime.UndocumentedPayload(headerFields: [(field: Server: Apple, next: 65535), (field: Date: Sat, 13 Jun 2026 02:43:54 GMT, next: 65535), (field: Content-Type: text/html, next: 65535), (field: Content-Length: 146, next: 65535), (field: Strict-Transport-Security: max-age=31536000; includeSubdomains, next: 65535), (field: X-Frame-Options: SAMEORIGIN, next: 65535), (field: X-XSS-Protection: 1; mode=block, next: 65535), (field: Age: 2, next: 65535), (field: Via: https/1.1 jptyo5-edge-mx-010.ts.apple.com (acdn/316.16515), https/1.1 jptyo5-edge-fx-028.ts.apple.com (acdn/316.16515), next: 65535), (field: X-Cache: hit-stale, miss, next: 65535), (field: CDNUUID: f0997761-ef15-4f9d-ac2e-03889f7307d5-156837625, next: 65535), (field: Connection: keep-alive, next: 65535)], body: Optional(OpenAPIRuntime.HTTPBody))) ``` but doesn't because the url changed to `6.4.x`