Adding custominstallerbuilder config_build.txt.

This build file is needed to set up the RepyV2 runtime which the
custominstallerbuilder requires to run. We need to do that because
plain Python `import`'s from Repy libraries don't currently work.

Future contributors, please check if this workaround is still needed
every time progress is made on SeattleTestbed/portability#25,
SeattleTestbed/portability#26, SeattleTestbed/portability#27, or
SeattleTestbed/portability#28!