gitlab-ci: Set the prefix to ./install instead of the DESTDIR.

If we don't set DESTDIR, then the DEFAULT_DRIVER_DIR built into the
libraries is correct and we don't need to use LIBGL_DRIVERS_PATH and
friends for CI usage.  Incidentally, this moves our installed paths
from /builds/anholt/mesa/install/usr/local/lib (for example) to
/builds/anholt/mesa/install/lib for simplicity.

Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>

.gitlab-ci/run-shader-db.sh

@@ -5,8 +5,8 @@ ARTIFACTSDIR=`pwd`/shader-db
 mkdir -p $ARTIFACTSDIR
 export DRM_SHIM_DEBUG=true
 
-LIBDIR=`pwd`/install/usr/local/lib
-export LIBGL_DRIVERS_PATH=$LIBDIR/dri
+LIBDIR=`pwd`/install/lib
+export LD_LIBRARY_PATH=$LIBDIR
 
 cd /usr/local/shader-db