`Runner.run` doesn't allow Bash builtins

Currently, the runner constructs a cmd to be run under env tool's environment.

env is not familiar with builtin bash commands such as cd. Hence, passing a cmd such as cd {$DIR} && ... will fail as it does not recognize the cd program.

Users implementing a custom runner will likely stumble upon this when using a staple internal such as cd. Which we'll want to prevent.

A potential fix is would be to construct cmd along with a hermetic bash binary which will run under env tool.