Allow extension of apptainer invocation via environment

The content of the environment variable APPTAINER_FLAGS is added to the apptainer call used to wrap the build/test commands.

This also ensures that the special directory /.singularity.d/libs is added to LD_LIBRARY_PATH in the container environment (see https://apptainer.org/docs/user/main/environment_and_metadata.html#environment-from-the-host).