ci: preserve existing LD_LIBRARY_PATH

Preserve an existing `LD_LIBRARY_PATH` when adding `/usr/local/lib`
to the CI environment.

The previous assignment replaced paths provided by the container
environment. On NixOS this removed the Nix store path containing
`libtpms.so.0`, causing the libtpms TCTI integration tests to fail at
runtime.

Signed-off-by: Juergen Repp <juergen_repp@web.de>

Author: JuergenReppSIT

.ci/docker-prelude.sh

@@ -14,7 +14,7 @@ source $DOCKER_BUILD_DIR/.ci/download-deps.sh
 
 get_deps "$WORKSPACE"
 
-export LD_LIBRARY_PATH=/usr/local/lib/
+export LD_LIBRARY_PATH="/usr/local/lib${LD_LIBRARY_PATH:+:$LD_LIBRARY_PATH}"
 
 # Change to the build dir
 echo "echo changing to $DOCKER_BUILD_DIR"