Refactor command executor

[mvgijssel]

This change introduced a regression to rules_task, that only a single defer kept working. The way defer currently works by wrapping a command with a piece of bash also feels a bit hacky, so maybe this is a good opportunity to rewrite the executor!

The executor should support

• multiple defer statements
• capture stdout to enable logging
• parallel executions
• capturing changes in env variables
• merging asts of nested tasks