.gitlab-ci: Use true --prefix

We have to use the final location of IGT for --prefix as some of the
binaries use that for asset lookup (e.g. register definitions that end
up in $PREFIX/share/).

To install it under a local directory during the early build steps it's
better to use $DESTDIR.

Signed-off-by: Arkadiusz Hiler <arkadiusz.hiler@intel.com>
Reviewed-by: Petri Latvala <petri.latvala@intel.com>