file(DOWNLOAD): Fix User-Agent to use run-time curl version

If CMake is linked to a system-provided curl shared library, the version
at run-time may not match the `LIBCURL_VERSION` at build time.  Look up
the run-time curl version to populate the User-Agent string.

This is particularly important since commit d3cbee99e3 (macOS: Prefer
building with system-provided curl, 2024-05-09, v3.30.0-rc1~130^2~1)
switched to building our official binaries on macOS against the system
provided curl shared library.

Fixes: #26209