fix: Send User-Agent header with cache analytics events (#14994)
### Description
Fixes #14993.
`POST /v8/artifacts/events` was the only Remote Cache request sent
without a `User-Agent` header. It is also the only caller of
`APIClient::create_request_builder`, which never set the header, even
though its preflight asks for `User-Agent`. Bot protection such as
Cloudflare Bot Fight Mode rejects requests from datacenter networks that
have no User-Agent, so CI runs never reported cache events.
`create_request_builder` now sets `User-Agent` the same way the artifact
methods do.
### Testing Instructions
`test_record_analytics_sends_user_agent` in
`crates/turborepo-api-client/src/analytics.rs` sends events to a mock
server that matches only requests with the expected User-Agent. The test
fails without the fix and passes with it.
