boulder: Customize script interpreter and version

[lumi-me-not]

Currently, the interpreter of scripts is hardcoded to /usr/bin/bash, but we might want to be able to customize this in the future. Even if only which bash version to use.

[ermo]

Could make the function have an overridable default that defaults to bash? So if there's an interpreter key in the relevant section (not sure which section that would be at this stage), we can use that instead?

[lumi-me-not]

Yeah, I also think defaulting to bash is a good idea. I don't know if we will ever have builds with a different interpreter, but it might be a fruitful thing to look into in the further future.

[ermo]

When we begin having "kinds" of recipes (= templates or actual declarative recipes, where we then save bespoke code in, say, recipe-library.kdl next to the recipe.kdl itself), then it might pay to have the option for e.g. ruby kinds to use ruby, or python kinds to use python etc.?