Consider embedding a shell

[Arcterus]

Currently we just start up sh and give it the commands to execute one by one. We do this in order to stop the build when a command fails. However, starting a new shell for each command can lead to some surprises like cd not working between two different commands. Embedding a shell would allow us to preserve changes to the environment between commands and (presumably) still check for command failure. It also might help solve the issue of poor variable substitution when deserializing the build file.

Possible shells:

• ion
• cicada
• The one we eventually write and include in MesaBox