Investigate why CI builds fail

[falko17]

Currently, the CI builds fail with no apparent cause visible in the logs, except for an exit code of 137, though the Unity CI script warns us that the error code may not be indicative of the actual failure. If we assume that it does point us to the error source though, this code apparently points to a lack of memory (which would explain why the build fails both on Windows and Linux and cannot be reproduced locally).

We currently have 12GB of RAM on see-ci1 and see-ci2, so we may need to increase this (though I don't know to what, nor why this much RAM is needed in the first place).

For now, the CI build step has been set as not required for merging pull requests.

Perhaps we can remove some old assets that we no longer need to remove the strain on builds.