Disable downloading for CI builds in cmake

[simonsan]

I tried to package offline and saw this todo "Disable downloading for CI builds". This is just another reminder for us. The buildscript will already patch EmbedPython.cmake and EmbedFont.cmake and make it unpack a pre-downloaded version though.

Changes should be made in:
openage/buildsystem/scripts/EmbedFont.cmake
openage/buildsystem/scripts/EmbedPython.cmake

It makes sense to stay with different options:

• Kevin: pre-downloaded->Path in filesystem -> (CI-flag)
• Script: patches newly downloaded repository on the fly
• Normal Repo: As is

So I guess, we want something like a flag for kevin e.g., to decide to use an archive offline, right?