gitlab-ci: Move cmake from prep to build command in csbuild
This is required to avoid csbuild scan failures when a commit removes
source files. The command prep is run only once before all the builds,
making csbuild to try to compile the old files using the configuration
files generated for the newest version.
Signed-off-by: Anderson Toshiyuki Sasaki <email@example.com>
Reviewed-by: Jakub Jelen <firstname.lastname@example.org>