[std] Add more functions for creating runnables

[kylewlacy]

This PR adds a few new functions to std used for creating runnables:

• std.withRunnable(recipe, options): Add a new binary in the recipe at the path brioche-run. The options include the command, args, and envs to run.
• std.addRunnable(recipe, path, options): Add a new binary in the recipe at the path specified by path. Same options as std.withRunnable, just with a custom path instead of always using brioche-run.

These new functions are meant to be a middle-ground between std.withRunnableLink() (where you can only directly run another executable) and std.bashRunnable (where you get a lot more flexibility, but need to include Bash as a dependency). By comparison, the new functions allows minimal templating for the command, args, and envs, but not the full capabilities of a whole shell.

Internally, std.bashRunnable now uses std.withRunnable() to actually build the runnable, which in turn uses std.addRunnable().