rand: stop detecting /dev/urandom in cross-builds

- Prevent CMake to auto-detect /dev/urandom when cross-building.
  Before this patch, it would detect it in a cross-build scenario on *nix
  hosts with this device present. This was a problem for example with
  Windows builds, but it could affect any target system with this device
  missing. This also syncs detection behaviour with autotools, which also
  skips it for cross-builds.
- Also, make sure to never use the file RANDOM_FILE as entropy for libcurl's
  fallback random number generator on Windows. Windows does not have the
  concept of reading a random stream from a filename, nor any guaranteed
  non-world-writable path on disk. With this, a manual misconfiguration or
  an overeager auto-detection can no longer result in a user-controllable
  seed source.

Reviewed-by: Daniel Stenberg
Closes #9038

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