(cmake): conan cmake script not correctly downloaded

What does this MR do?

• Check hash of conan.cmake file for:
  • avoid downloading if the file already exists and is valid
  • download the file if it's corrupted.

How to test it?

• Put a corrupted (empty) conan.cmake file in your build directory of sight, the file will be re-downloaded
• If the file is valid, cmake won't re-download it

Data

• no required data

Associated Issue

Closes #478 (closed)