[llvm][HTTP] Relax check for timeout error message (#209240)
When LLVM used curl to do HTTP requests, the timeout test failed even
though the request correctly timed out
(https://github.com/llvm/llvm-project/pull/188969#issuecomment-4960193683).
On curl, we prepend the error with `curl_easy_perform() failed: `
whereas with WinHTTP, the error is fixed as "Timeout was reached".
This PR relaxes the check to allow for the prefix in the curl case.